Not directly relevant but if you have the 64gb deck and a big SD card: you can symlink the directory that houses all the proton versions and shader catches to the SD card.
That was pretty much the first thing I did after I got my deck, as a stopgap before I could upgrade the SSD but it’s been working well enough I never got around to swapping out the SSD after, what, almost two years?
I didn’t do any fancy mounting, as far as I can remember. I think it really was just a matter of copying the directories to my SD card and replacing the directories on the deck with links to the ones I copied to the card.
I’ll take a look at what I did the next time I get the deck out and let you know