Do not recommend one software and/or one instance.
Using your scenario, would you recommend photography instances based on #Mastodon knowing Mastodon only allows up to 4 “attached” images? Not only that, Mastodon will only display 4 images even if there are more than that?
Or, are you going to recommend #Pixelfed designed for images. Or, maybe #Firefish, #Friendica, #Hubzilla, #Streams, which all allow more than 4 images and will display all the images even if it exceeds their software's attach limit?
Quite frankly, in my opinion, with the image display alone, Mastodon is highly not recommended. So, the number of users and instances Mastodon have does not make it the best #Fediverse software, as you have mentioned earlier, “Mastodon is the level of UX other projects should aim to”. It's not.
The best approach is we understand what the user needs and suggest to them the appropriate software and instances that will suit their needs.
Let's forget about the Fediverse for a while.
We have to remember that not everyone is on Twitter or Facebook. There are hundreds, if not thousands, of individuals who only have an Instagram account. Why? They don't need Twitter and Facebook.
In Korea, for example, they have their own culture and platforms for communication Twitter/Facebook-like, so they don't need those. But many of them have Instagram accounts.
Now, let's go back to the Fediverse.
If those are the users we are reaching out to, then there should be no problem recommending Pixelfed. Because for these target market, their only concern and type of use is to share, well, photos or their latest digital artwork. They don't care about a Twitter/Facebook-like experience or use.
That brings us back to the features of #ActivityPub. It is an “added benefit”.
Users who want to follow this content creator can do so using their existing account.
Okay, you can't do this with #Lemmy, the last time I checked, however, you can do it with #Kbin. That's a Lemmy limitation, not the fediverse.
For the content creator who migrated to the fediverse, in particular, Pixelfed in our scenario, they have a greater reach because they're federated.
(Extra: You can actually turn Pixelfed into a regular Twitter-like software if you are using the web UI. Although, last time I heard it will be removed eventually.)
(Extra 2: BookWyrm also allows Twitter-like feeds and interaction, it's not restricted to just books.)