Note: This advice isn't as applicable for Daggerfall and OpenMW, but I could help with those too, if needed. I am always glad to provide advice for modding Bethesda games.

When modding Bethesda games, especially on Linux, it's almost always easier to mod the Steam releases, as they are the de-facto 'supported' editions by the modding community. Other versions of the game, such as GoG's releases may have small or major compatibility issues and, while absolutely still servicable, may need more modding knowledge than a beginner can be expected to have. Doubly so for Linux, as most guides assume Steam + Proton (Though as an experienced modder, I've personally always used WINE).

The Moddinglinked series of modding guides are very good introductions to modding which aren't too long or complicated. Each also contains a small modlist mostly comprised of common dependencies and bug fixes, and are a great base to any modlist. The installation advice for MO2 doesn't really help on Linux, but otherwise the guides can be followed almost to the letter. I recommend following the 'manual' instructions over the 'wabbajack' for software compatibility and learning reasons. Linux versions of the guides also do exist, but I cannot comment on them.

Your best sources for mods is generally Nexusmods and, to a far lesser degree, the in-game 'Creations' menu. Outside of nicher mods, those two easily contain 99% of all available mods for PC.