@magpiemarket

I'm currently doing a deep dive on ActivityPub and the related specs with the intent of creating a PoC and, unless you have someone with you who has implemented ActivityPub before, I suggest you don't try to create your own back end.

Instead you should create a client that can use any sufficiently complete existing ActivityPub back end implementation. Concentrate on the UI and how you will extend the existing activity types to support your use case.

DM me if you have questions.