Great points. A signup that focuses on communities seems like a great idea, a lot of people will be looking for alternatives to their favorite subreddits. And 💯 on the accessibility problems with captchas.
Also, Mastodon's switch to making mastodon.social a default signup is leading to more centralization (signups have decreased significantly on other intances) and hasn't led to overall increased usage (total accounts and monthly active users are both relatively flat). So I'd be leery of using it as a model.