Thanks - some good ideas here! I've added QoL improvements to file management and saves to our list for a future update. In the meantime, there are some workarounds you can try:
First, if you open file explorer while the game is running, you can delete old files and Steam Cloud will pick up on that. You can always find the current save directory by mousing over the filter button in the UI (i.e., Auto Saves (Multi Player), or put this into your File Explorer address bar: %localappdata%/sins2
We've also changed Steam Cloud to only backup your last 10 files to save cloud storage space.
Another option - though I haven't tried this and can't say if it'll work - is to create a Symbolic Link to the sins2 directory in appdata and point it to another drive. You would do this via the command line like:
mklink /D "C:\path\to\new\link" "C:\path\to\original\directory"