I found this Python script a few years ago, it is simple enough that a non-programmer could understand 90% of it. I just remembered it had been running as a scheduled task all this time when I thought I had lost some saves.
Thought I’d post it here in case anyone can use it. Minor things are hard coded but they are few and easy to edit.
https://github.com/nonbhoward/backup_factorio_saves/blob/main/run.py
Ah yeah, I did that for a while until my save files started going over 200MiB, and I timestamp my base 4-5x per session, so I’m usually generating about 1-2GiB worth of save data per session (depending on the base). Steam cloud was no longer an option, so I had to fallback to some sort of manual solution.
Maybe Steam cloud is improved now? In that case I could re-enable it and have a double backup :)