Noting that there’s a third-party sync available for the Dropbox service…can’t remember what it’s called, but it was talked about on this forum and elsewhere.
So if the “nagware” on your system is bothering you, that might be an option. I’ve found that Dropbox’s back-end infrastructure is rock solid.
Recently OneDrive stopped syncing and while I tried to reinstall the app, it just would not let me ínstall the app from Mac AppStore! I fixed it eventually by downloading from Microsoft website. These kind of irritating issues at times. However, I have always found the fix.
Maestral:
Another vote for Synchthing as noted by @liminal. I currently use Resilosync to sync mostly text files from my windows to Mac. The sync is seamless and fast (even with larger non text files; this uses BitTorrent technology for the sync. Sometimes it gets confused with package contents and the syncing stops, but for single files in works well.
I am not sure how strongly the parent company is supporting the product.
Looking at their Web site, I think they are focussed more on enterprise rather than home. It was last update in 2022, but I did not know until I actually downloaded a version to determine the version number. There has been no additional features since I purchased it in 2020(not really needed).