Steam

I use Steam Link on the Deck to do this often. Personally I just leave it in 16:9- the Deck just displays horizontal black bars to get from 16:10 to 16:9. For me that's just part of the Deck experience because even a ton of games I play locally are 16:9 only, or even 4:3.

You can choose to match the resolution as well. On your host PC go to steam -> settings -> remote play -> [advanced host options] and look for something like "Match desktop resolution to client".

It also depends on what you are doing. I find that more resource-intensive games are often better streaming- better battery life with less heat and fan noise on the Deck. So changing the individual game settings to 1200x800 works well.

I don't use that just to access files though. I have several folders on my desktop set up as SMB shares and installed another file explorer (Nautilus) to access it. I mostly use it to move around PS2, GameCube, and Wii roms- everything else is either just permanently on my SD card or I'm able to regularly install via Steam.

For non-Steam game saves, I set up SyncThing to synchronize across devices.