Antennapod supports syncing with podder.net and gpoddersync

Have you tried https://github.com/jeena/fxsync-docker? It is a docker compose for selfhosting the new Firefox sync server rewritten in Rust.

Credits: https://lemmy.world/post/5839867

On Android nothing comes close to gReader Pro with The Old Reader as sync Backend. Sadly the app is discontinued, however the apk can be used just fine.