I wanted the deck to be a slim and portable thing so I added another user on my win10 machine, hooked up the external ssd to that, and then shared that folder to the additional user over the network.

I used the instructions here: https://www.reddit.com/r/SteamDeck/comments/wu044l/comment/il6xlti/?utm_source=reddit&utm_medium=web2x&context=3

to mount it as network storage on the deck. I just tried a bit of gaming on it and it worked okay.