I second this, custom domain on Proton, DKIM, SPF, DMARC all setup, have never had anyone complain about emails not coming through.
The only thing I'll add is that I did actually go through a migration from M365 to Proton and it was pretty painless, this was a while ago so the process might be better now, but I just exported a PST, used the bridge connector to add proton to Outlook, then imported the PST, once it synced I got rid of my Microslop licensing and switched to the official Proton Mail app, though lately I've just been using the web app since I haven't installed too many apps since switching over to Linux.