Hello! I can explain the 30 limit. Basically multi-communities are currently implemented client sided which means whenever you refresh, the client will manually fetch posts from every community and then turn those posts into a feed in a smart way that respects your sort order (currently multi-community sort is broken due to a bug but that's being fixed in the next release). This means that if you have 200 communities in a multi-community, the client will need to hit the server at least 200 times each time you refresh the post feed. To be nice to servers the client imposes a limit so that each user isn't spamming the server too much.
The way you are using multi-communities is novel and not well supported.
In the future, multi-communities should become supported server side so this limitation might go away.