as for “federated reddit” there is a lot of prior art here. on the activitypub side there are a few interoperable independent implementations, which threads crisscross:
https://community.nodebb.org/category/31/forums-and-threaded-discussions-task-force
using neither protocol, there is also an interesting one called plebbit that seems to maybe be a good fit for your user stories above:
“Group DIDs” exist in many other contexts outside of did:plc, but within PLC I think Zicklag already pointed you to most things that come to mind-- I know the Hypercerts folks did some group DID stuff, not sure what the status is on those experiments now that ATP Spaces™️ are en route?