Hi there, we came across this discussion and just wanted to recommend to take a look at Tutanota. You could move your custom domain accounts over, starting at €1 per user per month. Let us know if you have any questions!
I use tutanota after switching from Google with my custom domain back in 2018. It works great. I like the desktop and mobile apps. The improvements to the calendar have been solid enough where I am slowly migrating from a stand alone paid calendar app to it. I use cloudflare for my registrar and managing my domain records and the domain renewals are my only cost there. I pay €12 a year including taxes for just my one user account.
I set it all up once and with automatic payments and renewals a few years ago, I haven't had to mess with it since.
I really love that I can keep my wildcard domain inbound and use that to manage newsletters and various email services.