it seems antennapod recently got the play state sync feature using gpoddersync.

Nope, not realistic for “mirroring”. Federated could be possible, but I wouldn’t have high hopes about (good) latency and coverage.

Otoh, Spotify (and probably apple and other big corps) don’t even allow you to add RSS URLs, so I wanted to point out they Google was one of the big players which was more open.

Ah true. Companies are great at hiding the open web that they (ab)use.

Google Podcasts also supported entering RSS feeds manually.

No need for using sensational/clickbait headlines like this.

BBC has just started doing that at https://social.bbc

You do you.