I'm leaning toward Tutanota, but I can't claim to have experience with them yet.

Proton has appeared somewhat bumpy to me -- I can't say for sure why, but they give me some spidey tingles.

Migrating / transferring is indeed a problem. I would suggest using Google Takeout, their data export tool, and permanently archiving your data with a third party service and / or physical backups. See https://takeout.google.com/. You probably won't be able to import into your new provider.