I would really like the option to choose between multiple instances (even accounts on same instance??), which I think is the very last line of your post. :)
On reddit I was very grateful to the RES account switcher, and mobile app that let you easily switch accounts. I would probably never have actually gotten into reddit without those. It let me use completely different parts of reddit to express and explore different interests I think one of the things that made reddit the best social media platform was how pseudonymous it was. Total opposite of facebook "real name policy" attitude.
In the lemmy/kbin situation I am not sure how the interface would be. Need 1) some kind of persistent switching interface, 2) reminders of what account you are in. In RES it showed you your account name at the top of the page in the selector, and optionally above every comment box so you didn't accidentally post as the wrong account.
In the meantime I guess I will eventually install every available extension that does this and assign each one an account lol. Or pick one and make several local forks and install separately. hmm