You can learn more about my work here: https://github.com/verdverm/atproto
the community convos here: https://discourse.atprotocol.community/tag/private-data/2
At this point, Bluesky controls the protocol, decided what they want permissioned spaces to look like, and are not entertaining any other proposals (afaict).
I expect there to be an eventual successor that puts permissions at the core of the protocol from the very start.