it would be neat if you could not only group communities together, but also post to that "multireddit" and have the post appear on every community included in it. that way you can have a grouping of all the tech coms for example and not have to repost to each one manually.
I'm sorry but that's a horrible idea. What if I didn't group any of those communities into a multilemmy. So now I have to look at the same exact post multiples times? That's super cross-posting, which is in of itself just "legal" reposting to begin with.
I think this defeats the entire point of having separate instances. We need to start thinking of each instance's community as a separate space, not duplicates or mirrors of each other.