Textures are a little weird because they don't affect performance how you might expect. Once you load a texture into memory, it doesn't take any effort to keep displaying it. The main ways textures will impact performance is the time it takes to actually load them - typically through game startup or loading screens. In theory you can also run into stutters while trying to load textures on the fly, e.g. Skyrim loads objects in a cell system, so you're always loading the next thing in front of you as you walk. If your big textures get paged in and out too much you might notice it.

For the Steam Deck's purposes, this basically means that as long as you have enough VRAM to load all those textures, they're basically free to install. Bad things will probably start happening if you run out of VRAM though. There's probably a way to configure the MangoHud onboard the Steam Deck to show how much RAM+VRAM you're using at any given point. If you have enough headroom, I would feel free to install some texture packs and maybe give them a few stress tests in a city or etc.

As for "what mods should I install" that's a giant subjective rabbit hole. You could try a 1-click modlist setup from Wabbajack, or just use one as inspiration.

Yes, I've done it. It worked great after a clunky setup.

This video has most of the instructions, the rest are in the video description. I got it working and it's working great.

https://youtu.be/f0lC-alUAGg

Oh, and this is for Vortex only, so you could try a vortex collection I guess.

I did fallout 4 and it worked. Skyrim should be mostly the same and most Bethesda games have packs that claim better looks and performance. I used steam tinker launch to install mod organizer 2 to the prefix. You could do vortex the same way. I also toggled case sensitivity for the prefix folder in case of weird mod behavior, probably not necessary but it can only be toggled on empty folders, so before install or delete and reinstall, and if you haven't used the btrfs mod.