I agree about the user interface, but the rest I'm not too sure about. If you recommend new people just join lemmy.world then they really won't need to know very much about the Fediverse. The front page will work as expected, the Hot sorting algorithm will work okay and their instance will already be connected to most communities due to its size. However, you're furthering centralisation then, which is the dilemma.

Knowledge of the Fediverse and tolerance of clunky operations are mostly needed if you join a smaller instance. Learning to find smaller communities by searching for [!community@adress.tld](/c/community@adress.tld) (which doesn't work in all apps and sometimes needs to be done more than once) for example is not a good experience for newcomers, particularly those not tech-savvy. And sorting by Hot does not work well on a small instance.

Ideally the onboarding process could integrate lemmyverse.net somehow and give new users a suggested list of popular/active communities to start off with, so they can immediately find some places where discussion is happening without drowning in the memes of /All.

Don't bother with the Fediverse. Point them to Lemmyworld, that's all they need to know

Yeah. A good app is Thunder but still lacks key functionality like ~~editing comments~~ or proper Markdown rendering.

Also, if I see a link to another Lemmy instance’s post, I have no idea how to log in with an account on another Lemmy instance or another Fediverse service. Integrating OAuth-like Sign in with... functionality into ActivityPub would help a lot, especially if you get automatically logged into all instances whitelisted by your instance’s admins. However, that would require either a centralized server (which erodes Fediverse principles), cross-site cookies (which likely don’t exist for obvious tracking reasons) or a browser extension (which is browser-specific and an entry barrier for users, and could introduce security vulnerabilities).