Hi! The problem with that approach is that services like Nextcloud see the database as one single file. Therefore, even if you just mark a single episode as played, it has to upload the whole database again (20-100 MB). Additionally, it can not merge changes from two devices: if you mark an episode as played on one device, add an episode to the queue on the other device, and then synchronize, Nextcloud does not know what to do. In order to make a reliable sync service, a server is needed.
Gpodder is open-source, so it is theoretically possible to run your own server that is more reliable. That requires quite some technical knowledge and a server, though.
Unfortunately, I don’t have a better idea than that. Unless there is a volunteer who is really motivated to develop our own AntennaPod sync service, this is pretty hard to solve.