[jph00]

Your summary of the situation is not accurate. You can quibble about their choice of DMARC setting, but IMO it's the correct choice and best for customers. Your link says "Unable to determine STARTTLS status" -- which doesn't mean anything.

Fastmail has an excellent security track record. (Disclosure -- I'm the founder, but I haven't worked there or had a financial relationship with Fastmail for many years.)