Honestly your best method is going to be to make a new thread like once a week, in one of the major Lemmy-focused forums, until your userbase hits a figure you like.

It's going to be more effective than any other method by far, I think, as you're most likely to catch people right at that point where they're thinking "I want to experience a smaller Instance..."

Then they're gonna make an alt there, and if they like it, the alt will become the main.