I think “sub” is what people are going to call them reguardless. It is just internet language at this point, a subdivision of a community (by community I mean lemmy as a whole) is called a sub. Weather it’s a subreddit or sublemmy. I’m not saying bring reddit with us, I am just saying the internet can take the term “sub” with it and use it elsewhere.
Just call them sublemmies so we can just type subs.