Let me recommend Migadu, as email privacy is kind of a difficult topic. They offer complete email freedom for a very reasonable price; $20 ($10 for students) a year. They explain my main reasoning why I would avoid Proton:

When an email provider rations email address of your own domain name-space at a fee, they are asking you to hand them over control of your name-space. There is zero cost associated with additional email addresses and it is time you learn about it.

When email provider does not offer you standard email protocols that work with standard email clients, they want to lock you in for good. You are tied to using the dedicated applications offered by provider. The freedom of using a better or more suitable application is taken away from you. Protocols were standardized for a reason and today there are hundreds of email clients built for users with different needs.

When email provider alters messages data in non-standard format, they deny you data portability and with it freedom of changing providers.

Email is a collective effort of messaging interoperability. It is built around open, public standards and runs mostly on open source software maintained by folks believing in an open Internet, privacy and personal freedoms. Let’s not give away our freedoms for some Kool-Aid.