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.