Every playthrough I do, I install new lands mods like Bruma, as well as a bunch of follower mods, all of which I never wind up even going to or meeting cuz I tend to stick with some of the simpler followers such as those in 3DNPC or something like that, and can’t figure out how to work it into the roleplay that I go to the new lands

I have also never beat the Thieves Guild or Dark Brotherhood questlines even though I have had the game for over 9 years

I think the popular AI overhauls might allow the NPC characters to do that, at least AI Overhaul (https://www.nexusmods.com/skyrimspecialedition/mods/21654) says that it gives AI weather behavior, so maybe that could help you!

In terms of stuff that goes in the backend of the game, increasing the overall functionality I’d say SPID (https://www.nexusmods.com/skyrimspecialedition/mods/36869)

And also Base Object Swapper (https://www.nexusmods.com/skyrimspecialedition/mods/60805)

A lot of the mods that take advantage of these I find to be much better in terms of ease of use and compatibility for distributing new armors, abilities, weapons, etc. And a few older, script heavy mods like Immersive Armours and Wet and Cold have been given SPID versions if you’re still interested in using them but wanted less script bloat to your save

Now that I’ve played with it, I also really like Seasons of Skyrim (https://www.nexusmods.com/skyrimspecialedition/mods/62861) with the Turn of the Seasons or Seasonal Landscape add-on. It’s not too hard to install but for me it definitely helped me get better at modding, and in game in is really cool to see Skyrim in a totally new way. I would recommend you get some of those time passer mods like Time Flies (https://www.nexusmods.com/skyrimspecialedition/mods/39426) so that you can see the seasons a little more as this mod will allow time to pass while you do things slike read, smith, or sell stuff.