Last year I migrated from my hosting providers email service to Proton Mail after 20 years. I too was worried about preserving mails. I have used Thunderbird f

Last year I migrated from my hosting providers email service to Proton Mail after 20 years. I too was worried about preserving mails. I have used Thunderbird for everything, set to download everything and never delete, so had my primary backup there. I take regular backups of the profile directory as secondary measure, which are kept on my NAS and offsite. What I did was to just decide to do a hard break. I renamed the old IMAP accounts, added the Proton Mail accounts using the IMAP bridge, and then configured my DNS to point to Proton. DMARC and all that was easily set up as well thanks to Proton having nice guides and active verification. Now I still have access to my old mail in my old account folder, and I can use the Proton Mail app on my phone for new stuff. I also migrated a secondary mail, which was not using my domain and which I've had for almost three decades. There I had to do the laborious task of changing any accounts tied to it, and notify people still using it. I've been keeping it operational for a year and still get the occasional mail, but at a point where I'll be retiring it soon. Overall been very happy. It showed me it was easier than I feared as long as I have Thunderbird and mail accounts under my own domain.