That’s because performance is a criteria for getting verified:
default configuration: the game must ship with a default configuration on Deck that results in a playable framerate.
Source: https://partner.steamgames.com/doc/steamdeck/compat#DeckCompatibilityChecklist
There is no “target framerate” though, so what’s considered “playable” differs from tester to tester.
I have exactly the setup you described, a Raspberry Pi with an 8 TB SSD parked at a friend of mine. It connects to my network via Wireguard automatically and just sits there until one of my hosts running Duplicati starts to sync the encrypted backups to it.
Has been running for 2 years now with no issues.