Antennapod is great and also has syncing (through gpodder.net)

AOSP is still open source, it's just the source code is no longer released continuously and the reference device has changed from the pixels to a virtual device.

Google is still required to publish kernel sources, some of the higher level android code might be tricky but I'm not sure if the GOS team have said anything yet.

The grapheneOS team still got an android 16 release out relatively quickly