do I need to manually add all instances that I want future members of my instance to be able to subscribe to ?
seems not scaleable. I thought you could subscribe at one instance, then browse communities from all instances and read and comment no matter if your instance has it in their Allowed list or not ? (apart from the ones in Blocked)
My understanding is that if any user from your instance subscribes to a community, that community is searchable in the Communities list from your instance from then on... it doesn't have to be you specifically. Some user does have to find the community (likely via a search specifically for it) originally, though.
Discoverability definitely seems like a challenge.