I use gmail but I'm looking to find another free provider to move. I've considered proton or tutonota but neither seem to support thunderbird without payment.
It depends on how tech-savvy you are. I pay ~2€ a month to have an own domain hosted by "hetzner" (www.hetzner.com). You can create up to 100 Mailboxes under your own domain and are sure where your data is located. But that is not really viable for every person.
Thanks! For my personal domain I was using Tutanota, but I got tired of their app and really wanted to use my personal domain email along with the other I have in Thunderbird.