I think ProtonMail is as safe as it gets without setting up your own mail server. Their software is open source and they regularly perform security audits.
Price comparison:
Fastmail: $5x12 = $60/year
ProtonMail: €48 = $51/year
FastMail is 30GB, custom domain, infinite aliases
ProtonMail is 5GB, custom domain, 5 aliases.
For someone who uses IMAP and creates aliases when signing up / for filtering rules, as alternative to Google’s +addresses, FastMail looks like a better choice on the surface.
Can you convince me that ProtonMail is so much a better choice? Privacy matters to me, too.