But, I don't see why we have to assume this is the instance admins having an "agenda" and trying to "pressure other instances to limit users' freedom". Maybe they just want to run their instance their way -- the ultimate freedom!
[1]: I think I may have stumbled across the mastodon instance that you use. I can see why you might feel strongly about mastodon.art's policies.
I don't think it's about pushing an agenda. Each instance can only set the rules for their own community. Dozens of art-related instances have popped up like mushrooms overnight, so there is a good chance people will find one where they agree with the rules of the house. And it's easy to move to another instance later.
(I say credible, as in not just a tiny instance with no idea on longevity)
Welcome to the fediverse, where not everyone is on the same big server: There is wandering.shop if you're into writing. If you're into dinosaur paintings, go to sauropods.win. If you like math-art, it's very much on-topic on mathstodon.xyz. Try inkblot.art if you like non-conventional instances. Or genart.social if you like algorithmic art.
Those are just the ones I discovered by accident recently, without trying. Try picking a general instance you trust, and browse an artistic hashtag there. Also, most general instances are okay with art (try the #mastoart hashtag).
(Also the three instances recommended by .art when you try registering there. Probably not for you if you don't like .art.)
That said if the cost was a bit lower i was mildly tempted to standup a Blender community. However that seems like a lot of work, in addition to being a bit expensive.