I'm assuming most of us here want a large community.
Meh. It is not a big driver for me. I have been using text based forums since before slashdot. Some were big, like slashdot, most were not.
Personally, massive is not better.
I didn't use reddit because it was huge, I ended up there because to swallowed a few niche forums I used to visit.
Outside the small single interest subs, reddit reminded me of slashdot. I felt like a small voice drowned out by sheer mass of the crowd.
Yet, when I've seen federated software recommended on other social media websites, every article and many times, we tell everyone about the underlying technology first and THEN about how they actually compare to Twitter/Reddit
In marketing there is a term USP. Unique Sell Proposition. What does your product have that others do not?
The distributed nature of federated software is its USP.
"Why should I move from reddit/twitter/etc?"
It is social media that allows privacy and stops Corps selling your data is its USP.
If the person you are talking to does not care about the above, they have no reason to move.
You said as much yourself:
Telling them about tech-related things that they don't know about or aren't really interested in doesn't help much.
The problem is that in all other aspects of social media: ease of use, userbase etc the various flavours of federated social media are last.
TLDR: If the average user does not care about the technical reasons federated social media differs from the rest. They will see little to no benefit in switching.
The local artist down the street likely doesn't care about having a deep understanding about federation works or the benefits of decentralized versus centralized social media. They just want somewhere to post their art for others to see and comment on others in their space.
Personally, I am a bit sceptical about the long term sustainability and scalability of data storage for data intensive (images and video) federated services.
If they see a large influx of users and usage, the hosting costs are going to rise fast.
Unless the artist you mention is willing to set up and host their own instance and that is another jump beyond getting them to create an account.