if you want to talk about difficulty, you have to mention Mort's rebalances first imo.
How's this then, check out my difficulty overhauls setup my fren:
Money:
Buying Game Updated and
Smarter Harder Barter: overhauls the economy, makes mercantile investiment actually useful
Leveling:
Proportional Progression Expanded,
Expensive Training,
Magicka Based Skill Progression: makes leveling slower (and customizable): no more spamming to level up, no more gaining 20 levels after raiding one tomb, casting big spells is more rewarding training-wise and less of a chore of spamming cheap "training spells" later on
Honorable mention for leveling mod:
herberts literacy program overhauls how skill books work. Instead of giving you 1 point for the skill, it gives you multiplier bonuses for damage, xp gain, rolls etc... Not so great for vanilla, but excelent mod for playthroughs that make leveling slower.
I started a new playthrough around monday, and i'm still level 3 today. If i want training to get my destruction from 30 to 31, i'll have to cough up around 4k gold lmao
Loot:
Realistic Repair: though it may seem i'm recommending this for the "repair only at anvil" feature, it's not. This mod makes so that when you kill a NPC, his armor and weapons won't have full condition anymore. You've gotta fix them now if you want to profit from it. Classic example: that quest on Tribunal where you get a free High Ordinator full set from a dead guy (not to mention a full glass set). It's just too easy to sell it all for around 20k gold even with all price balancing mods.
Alchemy:
Controlled consumption: no more chugging 3000 sujamma and killing vivec with a slap to the face. You can chug 4 sujamma at once and that's it (that is still a lot extra cheap strength)
The Art of Alchemy: no more brewing 100 potions in ten seconds. You now need to place apparatus somewhere if you wanna brew, and it takes real world time. Some ingredients need to be grinded on the pestle if you wanna use them. You can use calcinators to filter offnegative effects from your potions or positive ones if you're making potions. The alembic can be used to concentrate effects from two similar potions (capped with your skill).
The result is that you're no longer able to brew gamebreaking potions, and makes the process more immersive. Makes you feel like a real alchemist brewing stuff.
I think those mods makes the game more difficult without making it too grindy: raiding tombs are dangerous, easy cash from looted equipment is not that easy anymore, you'll get more excited finding gold and potions, cash rewards from quests will actually matter. Speaking of cash, if you're really on that MUH IMMURSION pack, you can get a mod that makes gold actually have weight. I did a run like that once and it was painful...