if you want to do selective upload, RoundSync works - it can create “sync tasks” too to do both: syncing and selective upload.
It works because roundsync asks for the permission to manage all files and doesn’t go through a SAF mount (the latter, as I surmise, doesn’t give the library doing the file upload the file size, thus needing chunked uploads, thus failing at the fcgi part of nginx-fastcgi-php that powers most nextclouds, it won’t accept content-lenth: 0 - apaches can).
Edit: apparently, RSAF (a rclone wrapper too as RoundSync) can do both: act as SAF webdav mount and has working copy’ing to it. Recommended.