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In what regards are Fastmail and Gsuite the only "real" options for email on your own domain?

There are tons of other options. Mailbox.org, Migadu, Protonmail, Tutanota and Mailfence for example.

I've been using Posteo.de for a couple years now.

Great service, E2E for everything, including contacts and calendar, and it's quite cheap (I think I pay €12/year).

You can't use your own domain though, AFAIK. Which means you won't be able to switch providers, which may or may not be a problem.