This review is written after Volume 4 Canto I, as it is the latest volume translated to English at the moment.
Mostly spoiler-free. I'm avoiding all the big events, but don't mind talking about small details even if important. And this is mostly just an incoherent ramble, anyway.

Min-Maxing My TRPG Build in Another World (Preach the Good Word of Mr. Henderson) is, as the name would suggest, an isekai that takes place in a TRPG world. TRPG worlds are, in my opinion, much more fitting to be placed in story format than a video game world is - for one, you can find replays of actual TRPG campaigns that people have played sometimes, and they are often very fun reads. The main difference that makes it fit well in a setting like this is that the TRPG emphasizes freedom - while there is a main plot, you are free to ignore it and simply do something else, causing the DM¹ to suffer as they figure out what they're supposed to do now that they haven't prepared anything that works for the current situation. Plenty of isekais simple take place in a world with RPG stats rather than a game world, but it's always fun to see a world that has the stats but doesn't tell the player about them.
While obviously not based on any particular TRPG, the world is written in a way that it's believable that it could exist as one. Well, that's why it's a TRPG world.

The main character, Erich, gets sent to this world by an aspiring Buddha without any goal at all, instead just being told to do whatever he wants. Of course, that (aspiring) Buddha did the unforgivable: They gave Erich a cheat power! How are you supposed to enjoy the story now??
But the power he got is one that isn't strong enough to ruin the story, and instead feels like part of the story being a TRPG world. Sure, maybe it'd be nice for the others to also have the "allocate your experience to whatever you want instead of needing to train something related to that thing" ability, but it's a staple for all the PCs to have that. And it's not that strong - a few people give him weird looks for using it sometimes, but most of the time you just upgrade the things you're actually using regardless, so it doesn't impact that much in the long run. And most of the companions are about as strong as he is, anyway. It just isn't fun when you steal all the spotlight, like when the cheat power is actually OP. (Not that those stories are always bad; there's a few I find decent.)

The story is formatted in the usual format where an event begins and then it slowly builds up to a climax, usually along with one of the waifus (I'll get to that later). It feels like with a deranged enough group, a campaign like the one shown here could have happened as a real TRPG in the fictional framework that it occurs in. Part of it is due to the main character likening things to dice rolls, of course, but there's a character who acknowledges having a (accurate) spidey sense for when a crit fail is about to be rolled, and even if they never say it that way in story (as someone who hasn't gotten isekaied, they wouldn't know to call it a crit fail), it's still clear that this is what it's supposed to be. It also includes classic-style dungeons as a matter of course, and many other common tropes (there's been plenty of cursed items through around already).

Any world that has waifus I like has got to necessitate talking about them. And might as well include everyone else notable while I'm at it. The character dynamics mildly remind me of how harem stories tend to go, but by keeping the harem small and having them like the MC for a reason other than being absurdly strong ~~instead it's being handsome~~, it makes it pretty nice because I've been reading too much trash lately.
What I really like about the characters is that a lot of the story is done through them. I often care more about side characters than I do the mains in any story, but in this one, almost every moment of the plot is based on at least one of the side character's stories progressing as they begin to intertwine with each other after they meet (a requirement for a really good story, in my opinion). While undoubtedly MC-centric, he's not too important but still gets a modest amount of development as time goes on. Maybe it's because I really can't self-insert into someone who hates vitality glorifiers (lolicons), but he doesn't feel like too much of a self-insert.
The story starts off with Erich's childhood, and associated childhood friend Margit (a loli arachne, with previously mentioned spidey sense), living the countryside life as so many isekais (that start the MC not-OP) do at the start. She tries to trap (ie. she's a spider) Erich into becoming her lover. It's not a character I actually like that much.
His younger sister Elisa is the cutest girl in the world ~~(it shows in the artstyle, hers are the only pictures that are actually pleasant to look at)~~, according to Erich at least. Having Mana Capacity rank [REDACTED] (according to her character bio), her arc hasn't even really appeared yet, but she's a vehicle for slightly more things than I'd expect. While Erich's fey friends (one who likes trickery and one who's wholly innocent) are pretty bog-standard for their archetype ~~this is how I'm ruling out Ursula as a waifu candidate~~, her introduction came with a really impressive scene that really didn't amount to anything except for the main character gaining a treasure... and yet it's my favorite part of the whole story. It feels like a big moment while reading it. Maybe it just boosted his desire to save his sister even more than before. Either way, her very strong love for her brother is the sole motivation for her to do stuff, which is very sweet. (I ship them. Maybe I do just have a preference for sister-based novels?)
Maybe it made him appreciate Elisa's magic teacher, a methuslah (read: elf) a little more for going to help his sister. This story talks a lot about the monotony of immortal(/long lived) life and how it affects one's values compared to a more fleeting life; while a common subject, it's one I don't tire much about reading whenever it shows up. I enjoy seeing the large amount of individuality each character is given, even the more minor college professors and the like, because there's really no completely normal person out there and it shows by how literally everyone important has at least one eccentricity that would be annoying for those around them to deal with. Which is also a common thing to see in most novels, but none of these eccentricities feel annoying for the person actually reading the novel. Must have to do with how people usually have some known motivation behind all of their actions instead of acting impulseively. The elf gains some sense of taking care of her charge over time, but even when she was first introduced we were given plenty of justification for why she acts the way she does. People who think outside the standard good morality framework are always nice to have.
And in addition to that, there's the lolicon dean who simply likes to dress up cute things. Like Erich (who avoids her as much as he can) and Elisa (who goes along with it due to not being competent enough to avoid her). They did a lot of worldbuilding about college politics and the like, probably to not much use other than that it's cool to have it and could come in handy in the future. She's not actually very funny, but most scenes that involve her make me laugh, so maybe she's the comic relief character (despite being, like, the big boss)
There's also Erich's college friend Mika, who has questionable enough gender that they're referred to with different pronouns depending on the situation. The excuse they made for that aspect is absolutely stupid and that makes it my least favorite aspect of the story so far, but she's already gone on some adventures together with Erich and is bound to continue doing so, since they're friends and all. Even if they might not stay together once Erich finally lives the adventurer life of his dreams, since she's the sort of character who never gets seen as a love interest; not that I'd expect the romance to ever be a major part of the story.
There are, of course, many more characters who take the spotlight at some point or another, but unless I can squeeze some other info into someone's paragraph, I don't see the point.

The story contains the Henderson Scale in each and every table of contents page, full of examples which sound like they really encompass what a TRPG campaign is about. (The examples change every volume; the scale does not.) But the main thing to note is the bonus chapter in each volume, wherein they cover an AU that could potentially have happened should something different have occurred in one of the critical choices of the campaign, which if chosen would change Erich to a large enough extent that the rest of the story wouldn't be able to occur. While I'm used to short stories being interspersed throughout a light novel, I haven't seen any in AUs before, and I find it interesting how little they need to establish what happened to cause the events to happen, and how they can cover a fulfilling story despite the overall irrelevance with the rest of the story.

This rambling has really been going on for too long, so I'll stop here. It's been a while since an isekai has made me really want to see what'll happen next², which I think is a sign that, despite its flaws of the main character being a girl magnet, I still really like this story.

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¹ At some point, this story says that you can tell what someone's preferred TRPG is based on what they call the GM/DM/etc. I had never really thought of this before, but thinking about it, it's very much true.
² There are plenty of stories that I just read because I've been reading them and don't see a reason to stop since the story is good enough to follow it, but even for something like my favorite isekai (bookworm), I'm only looking forward to when I'll be able to read it, rather than specifically wanting to see what'll happen.