When you're creating a sublemmy, if there was a checkbox for "create a matrix chatroom with the same name and description as this sublemmy", would that be a good feature?

I think yes because having to manually go and create a room is more time consuming, but the trade off is having dead sublemmys creating dead chatrooms, particularly because domain camping on matrix is a problem. (ex, I bet #sex:matrix.org is a room on matrix with minimal/no activity)