How do you request additions to starter packs?

That’s a good intro text. Mine is similar.

Who manages this, and who defines the criteria?

You probably want a single URL like join-lemmy.org that manages the selector.

Would likely need some algorithm based on geography and referral data, with some amount of randomisation.

There is no simple solution, but removing instance selection (or any discussion around federation) is a must for on-boarding.

Mind you, I do think there is value in talking about the benefits of federation - e.g. “A single entity cannot buy out Lemmy, it is designed to withstand such takeovers and remain independent”, but not about specific when people are just joining. The ones who are interested will figure it all out themselves.

An automated default instance selection is a must.

I would even go as far as removing the additional selection screens. Just have a small “choose custom instance” button with a “feel free to ignore this” clarification text.