I had a quick look at the instances on joinmastodon and found these rather fitting as well from their self-description:
Those are quite small in terms of active profiles as far as I can see, but e.g. fairmove.net is run by the fairkom association who also provide lots of other services (that were known to me before). And I have to admit I see a plus here because they also offer a crossposter to Twitter, which I personally would prefer to use (Mastodon to Twitter only, definitely not the other way round; as it is the case with the current account and some other crossposter service).
I agree a lot with what Urs said but I don’t really like the idea of switching to mastodon.social as I personally would really prefer to be on an instance with a focus on Fairphone’s values. To me, mastodon.social would feel like a wrong choice.
Urs also raised a good point on the payment issue. I hadn’t given that too much thought so far. One of the instances I looked at said they ask for at least €10 per year. And when I read that my first thought was “well, I’d happily pay them €12 anually from my own pocket if all else suits us fine”.