Personally on Gmail, but been planning the switch to Fastmail for some time.

I’m willing to pay the $50/year ($4/month) for their 30GB plan to get my email out of Google’s hands.

I particularly have concerns over Google deciding to lock out my whole account over something stupid, like keeping ebooks I’ve purchased in my drive for easy access on all my devices (though maybe that is a symptom of using them for storage as well), or some other arbitrary reason, and losing access to all my accounts elsewhere all over the internet.

Much rather have an email at a custom domain I have ownership over, so no one can ever keep me from using the email address.

From all I hear from IT professionals about it not being worth it even for them to run their own email servers, I’m willing to pay a company for that, and the privacy that comes along with a professional business-client relationship.