Hmm so I would say go with the switch 2 mainly because you mentioned money isn’t an issue. It’s going to be purely peace of mind as you’re not going to need to do any major troubleshooting later.

Alternatively just go with the steam deck and let her use it normally and (I assume as I’ve never owned a steam deck) it should just be plug and play the same as a switch.

Coming from experience, if it was me and my mom in the same situation, I would go with the switch 2 because I know there would be less stress involved overall. I normally do the tech help requests for my parents’ pc’s and phones, and it always surprises me how they can sometimes manage to break things or get themselves into a pickle.