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Why I ditched Gmail for Proton Mail
(www.androidpolice.com)
This is a most excellent place for technology news and articles.
It certainly can be a bit involved. When I moved from Gmail address to my own personal domain I did it slowly over a few months.
I set my Gmail address to automatically forward to my new email address. Then I setup a quick filter which added a label on everything that had been forwarded. Once a week or so I would look at all the emails that had been forwarded and update them to my new email (or delete them if unwanted).
Did the same thing. Took about a year... Just do one site at a time.