I’ve been using fastmail for a few months now. Besides being a really top-notch replacement for gmail and even adding a few features, it also onboards you by giving you a couple quick fields to fill out and then immediately imports everything you had in gmail. And then it keeps importing it, continuously, as long as you want.
It’s not like i’m paying for gmail, so I’ll keep the account alive as long as I need while I switch all the same accounts and records that you are (validly) mentioning as a barrier. I could actually do most of it in one sweep, if I just searched for my old email address in 1password, but it’s not really that time-sensitive to switch, and in the meantime I get to use fastmail.