Approaches that might work better involve integrating a good instance picker into the signup process, or an onboarding flow that treats the initial instance as a "starter instance", a base for exploring that makes it easy to move to another instance. Realistically though it's not clearly how likely it is that Mastodon gGmbH will prioritize the work that's needed to support either of these -- which isn't an argument against pushing for them, just that we should be looking for other alternatives as well.
In general it seems to me that might be better to focus our efforts in terms of making it easier to join communities in fedi as a whole, not just Mastodon. For many people something other than a Mastodon-based instance may well be a better choice. Of course that still leaves the problem of people who search for "mastodon", or have read an article about Mastodon and followed the links to either the Mastodon app or joinmastodon ... but I don't know how to address those without Mastodon gGmbH's cooperation,