If they want to move r/rust to Lemmy, then just pick an instance and move, or start your own instance if none of the existing comms matches their needs, instead of all of these excuses.

Again, it's really hard to get this through people's head, but Lemmy is not a reddit clone, every instance is an independent forum each with their own different policies and moderation. If you don't like the comm of one instance, you can easily move to a different comm on another instance with the same topic.

For different comm on the same topic to follow each other would honestly be a moderation nightmare, which is why I'm only supportive of user level community grouping (which would be more along the line of having multiple subscribed tabs. )