IIRC migadu.com is $9/month, but that includes essentially unlimited domains and mailboxes (but only mail, nothing else, which seems to fit your use case).

Which means, if you like to set up new domains for specific projects, there's no extra cost (other than registering the domain of course).

Have never had a spam problem, but they stress that they don't consider themselves mission critical.

I went with fastmail for my personal email, after a small-ish provider I had used (and liked very much!) between 2001 and 2018 was unable for a few month, in 2018, to avoid getting my mails sorted by gmail into spam -- no delivery problems since mail. Alas, despite wishing to support the smaller players, silent non-delivery of emails is a huge problem.