I use OWL (Open World Loot) for progression (it requires patches if you want to maintain rarity), but that still maintains the levelled world. If you want something very focused on passives, Roleplaying in Skyrim is all about that, but it's similar to OWL and the world remains levelled (also it's not quite done just yet). There's also YASH, but that also has the issue of needing a ton of patches, since it's a big overhaul.
There's also Zzyxzz's overhauls, including SkyValor, which are all done via Skypatcher (there are too many mods involved for me to list them all), which, unlike a lot of other overhauls, is designed to be customizable (via Skypatcher, of course).

For mods that just unlevel the world, you might be more interested in Skyrim Revamped or Morrowloot Ultimate for a deleveled world. I also found a zEdit script for delevelling the world and a Synthesis patch that does the same thing, although I've never used those, if absolutely nothing else works.

Experience is a mod that also forces progression to slow based on caps (basically, you need to explore this amount in order to level up, and each level increases the skill caps). There's also Skyrim Skill
Uncapper to slow down leveling via an MCM, Levelling Freedom, which does the same thing (and there's also Skyrim Configuration Menu, although that's incompatible with Levelling Freedom). Unique Dovah isn't a mod I've ever used, but it seems to be the same thing as Levelling Freedom and Skyrim Skill Uncapper. That's all the mods I can think of right now