Are there any gpodder servers that are not the official ones?

I use the nextcloud gpodder integration myself with antennapod (phone) + kasts (desktop) and it mostly works well with subscription sync and timestamp sync. It'll occasionally forget that I've listened to an episode on desktop but I guess that's just growing pains.

I've been keeping an eye on podfetch as a potential nextcloud-gpodder replacement but it's not just there yet for me.

I also just saw podcast merlin added to the supported list at https://github.com/thrillfall/nextcloud-gpodder but I don't use windows so I can't speak to that.

As for finding an alternative to the sync server, my only suggestion is to find a free/paid option at https://github.com/nextcloud/providers#providers that'll let you install the gpodder nextcloud app or use the oracle free tier to host something light like podfetch.