As long as tools to unclip the shell aren’t consider specialized, I think almost all existing handhelds are gonna meet the actual requirements here - they just have to be user replaceable, not use swappable, without the use of specialized tools or thermal energy. If you can unscrew it, disconnect the old battery and connect the new one, it complies. It’s really only an issue in waterproof devices, where they have to glue everything to seal it.
Unless something has changed very recently, Destiny 2 does not work under Proton because the Anti-cheat isn’t compatible. Windows is an option, but I’d really recommend a dedicated Windows handheld for that instead of the Deck.
I haven’t tried Windows on the Deck itself, but based on 6800u performance, you’re looking at maybe getting a solid 30fps on lowest settings. Destiny 2 is not a very well optimized game atm, and even the Ally at 20w can have trouble holding good performance in Neomuna or the Tower.