How to make legally correct storage of accounting on backend-e?

Question

User identification

On the client, users are authenticated by email, which is personal data under the Fed and/or other legitimate GDPR and other legal regulations. That is, you need to do a bunch of actions (notify, get permission for third-party storage, have a headache with the location of the server, etc.).
If I make a one-to-one conversion 'abracadabra' = func1('[email protected]') and have '[email protected]' = func2('abracadabra') and on the backend server all custom bindings (among which there is no personal data - ala Full name, maps, addresses, places of birth - only the UI parameters of the application) I produce to 'abracadabra', then from a legal point of view everything is clean (https is used)?