That's the thing

Every new user should by default download the official Mastodon app and make a mastodon.social account.

Any other choice should be under an expandable Advanced or Custom Account Creation.

Techies claiming to have to idea what the problem is are really revealing a serious lack of intelligence.

People are primates. Chimps with language.

You've seen the video of the chimp browsing Instagram. That's what the user is like. That's how easy you need to make the UX.

Largely, they're clueless with simple motivations. They don't care about helping the fediverse be a viable and growing competitor to corporate social media. They don't share your goal, they don't care, they won't suffer through even the slightest inconvenience for this.

So make the decision for them. Don't present them with the list of choices unless they specifically ask for it.

Put horse blinders on them.

They need it.