[xeo84]

You can send email from your alias with the Mail app, I do it every day. You need to set it up manually (not use the certificate from fastmail). After you set up the email address, go to Settings -> Password and accounts -> Tap your account -> Tap you email -> tap the "Email" field and then you can add your aliases tapping the button "Add Another Email"

[adiabatty]

I'm pretty sure you can add e-mail aliases even after using the certificate from Fastmail.

(Disclaimer: this is undoubtedly true on macOS; I haven't done this sort of thing on iOS in ages.)