Making your decision based on that doesn't make sense, because it doesn't affect FastMail at all. They're not an end-to-end encrypted service. They can already access all of your data without a backdoor (and need to be able to, to provide features like search).
I've been using posteo.de. I haven't heard anything negative about them, they're not terribly expensive, they support IMAP (unlike protonmail), and support is excellent (had issues syncing contacts to a BBOS10 device they solved immediately)
More info in this blog post, specifically under "The AABill doesn’t change your privacy or data security with FastMail": https://fastmail.blog/2018/12/21/advocating-for-privacy-aabi...