Ah I’ve looked into such things before and everything I found started with installing tons of extra software to customize the “gaming mode” UI first. So I landed on just using SMB. I tried adding Dolphin (the file browser, not the emulator) as a non-Steam game but it didn’t work on gaming mode. When I was younger and broker such software solutions were more attractive. A larger internal SSD is the more expensive, but also much simpler and more elegant solution for me. I DO wish there were more basic utilities like a file browser built into the gaming mode OS. And for all I know maybe Valve is looking into that.
Also I use SyncThing to sync my saves: I like how it’s all just stored locally and works on Windows, Linux, and Android. And it works pretty well as a non-Steam game in gaming mode on the Deck.