Oh, sorry, I have my own domain so I just changed my DNS, imported my messages to Fastmail and that was it.

I did it little by little, no need to rush it. Just start changing some main ones and as you log in to the others take the min it takes to change it.

On top of that I moved services to their own custom email like <service-name>@<my-domain>.com. That way it's all neatly sorted inside Fastmail in different folders (and I know who is selling my data)

My approach has been simply signing up for new services on my non-Gmail addresses, and occasionally switching important ones over. Nobody's making you delete your Gmail account, so you can still keep it open for legacy stuff as long as you need to (especially since it's free).