I admit RoundSync has an error prone nextcloud auth setup and isn’t as easy to grok in concept. Until that is solved (there are remaining issues) I shouldn’t wave its flag. It works if one has an preexisting rclone config to import, but it’s convoluted.
A commercial alternative is FolderSync.
But back to your deletion issue: I think there’s no way around “cleaning up” on the cloud end to get under quota again.