Since you'll get more exposure:

My complete onboarding solution pack would be:

1. Upon account creation, automatically follow 1500 random accounts. You can have a button in the client that says "Follow 1500 random people". Then, start curating your feed if you don't like what you got.

2. The most useful feature I found when I first got on: "FOLLOW ALL" button when checking Jack's follow account, and subsequent few accounts. In ten minutes I was following over 1700 people and had a populated, lively feed.

3. The aforementioned mirrors/bots

4. Automatic relay management, configurable later on as the user gets more knowledgeable.

Stuff like asking people to pick tags when they create the account and whatnot, is not useful *yet* because the content simply does not exist. What's the point of me picking a bunch of tags that deeply interest me like "linguistics", "Catalonia", "seafood", "knives"... if there's nothing or nearly nothing about any of them -- yet? In the future, yes. But right now it's not worth wasting time on this.

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Even with all the above, we all know that the UX is, at best, wonky. In my opinion, it's just not normie-ready yet, and expecting to have "normal" UX retention rates is delusional. A shortlist, and I'm only talking about "social" clients:

Devs haven't even sorted out a single common way to 
quote events. Some use "@", some "nostr:", some nothing... It's a 
shitshow, for being a "protocol".

Embedded content display is not smooth and varies greatly between clients, especially video. The system to embed content is not coherent among clients.

Clients not only don't talk to each other - they nuke each other. The fact that we advertise "one single account" for all apps. Yet, the simple act of logging on through another client may cause you to lose: 1. your follows list, 2. your relays, 3. your own fucking notes. Kind of bums people out...

Right now I have to switch between Damus and Coracle to write this because the some notifications and notes are missing from each other. 

Search is terrible, when it even exists. Clients like Damus don't even try.

The list of UX-related stuff is really very long, and I'm not one of those who bitch about silly shit like "oh, it doesn't scroll up and down smoothly". But in any case, as I said before, the main and most pressing problem is simply populating one's feed.