I believe you don’t get Shadowplay without it, so if you need game recording / replay without impacting performance it’s worth installing. But I would recommend the nvidia app beta instead of geforce experience as it doesn’t require a login.
Other than that you don’t get any optimizations by installing geforce experience, other than its game settings tuner thing which doesn’t work at all.
An SSD upgrade to 2TB or something would cost less than the game does these days. Or if they’re on an extreme budget they can probably find a basic 500GB or 1TB SSD for $10-15 used somewhere.
I feel like windows would seriously hamper the experience, it’s terrible on small screens and when you don’t have a normal mouse and keyboard to use. It also has a ton of overhead that’s going to eat into the more limited CPU and GPU.
I’m also pretty sure you can just dual boot windows on the steam deck if you want to have windows available.