I'm not sure if I've properly explored the idea, but a specific "digital culture" would help a lot. Inside jokes, ways of speaking, emojis, memes, etc. really help an instance to distinguish itself from others.
The best example is Hexbear (ik but just hear me out for a second). Their culture is borrowed from the edgier side of the leftist internet, but they still have a style of their own. They were so recogniseable even, that a user claimed to be scared of seeing pronouns next to someone's username because they knew it would be a comment from Hexbear (they used to be the only instance with such features, before others followed suit).
I have a hypothesis that a good amount of issolation, or at least encouraging users to only post on communities on your instance, would be good for developping some kind of culture, maybe even kinship between them.
shhhh! They haven't finished building their strawman