@Imaro, just to be clear here. No, I'm not looking for a way to make Madness and Sanity rules work in D&D. I tried. The mechanics in the DMG did not work for me. Like, at all. The players, as I said, absolutely hated them and, well, that's largely that.
In fact, that's largely my take away from all of this. People keep going on about how easy it is to change 5e. That has really not been my experience, either as a player or as a DM. Every time I've either played or run D&D, and tried to move away from baseline, it has largely resulted in failure. I've now talked about running infiltration scenarios, low magic and horror. I've DONE the last two of those and participated in the first under three different DM's. It's never worked.
So, you're right, in a sense,
@Imaro, that I'm not really looking for ways to make this work in D&D. D&D, AFAIC, is such a tightly bound system that I simply wouldn't try any of these ever again in a D&D game. There are so many knock on effects to any changes that it is not worth the effort to try to pound the square D&D peg into round holes. It's simply a recipe for disappointment, in my experience.
Which is why telling me that it's somehow wrong or mistaken to tell people that running these things in D&D is a waste of time just flies straight up my nose. I WISH someone, years ago, had just pulled me aside and told me that when 5e came out, it would be best to stick to bog standard D&D tropes and concepts. It would have saved me a LOT of frustration and a ton of time and effort.
See, the funny thing is, my current campaign is pretty much that. Bog standard D&D. Couple of minor class alterations, but, nothing that particularly comes to mind. And it's been a riot. Totally fun. Fun for the players and fun for me to run. So, telling me "Oh, it's
easy to change 5e" is the absolutely worst advice you can give. Because
in my experience 5e is anything but easy to modify. It's really, really HARD to modify outside of a couple of areas. New monster? Ok, that's no problem. New class? Maybe a bit of work, but, so long as you keep to the math, and don't mind some rough edges, probably not a problem. Drift genre and try to add major mechanical changes like a Sanity/Madness mechanic? Don't bother. It's a waste of time unless you have players who are REALLY into drifting D&D into different directions.
Otherwise, just play a different bloody game. It's far easier and, again, in my experience, your success rate will be much higher.