Keep in mind that ATT applies to advertising tracking. If you want to collect a user's email for the purposes of eg. internal analytics or promotion (like sending them a welcome email, or sending them a special offer) then you're really only bound by GDPR constraints. Exposing the user to a GDPR consent form is probably more of a necessity than exposing them to an ATT prompt: you can skip the ATT prompt altogether if you don't plan to collect data for the purposes of ad tracking.
Note that fingerprinting is not allowed under iOS14 guidelines if the user has not opted into ATT.