I switched from Gmail to FastMail about 4 years ago and have been very happy with it. At the time, FastMail had no problem importing a decade's worth of emails from Gmail, and I still have the deprecated Gmail address forward to my FastMail account.

I'm on their ~$50/year Standard Plan so that I can use it with my own domain names. I point my domains to FastMail's nameservers, and it automatically configures all email-related DNS settings. It can be setup to accept mail to arbitrary addresses, so I now use service.tld@my.tld when I create accounts on other websites, which can help with filtering or seeing who is leaking my info. Any other type of DNS record can be manually added, so I can still generally point my domains to my GitHub pages.

I can recall only one outage since I've been using the service. I use their web client and Android app (which is just a thin wrapper around the web client). They are not glamorous, but they are functional and feel snappy.

One thing to be aware of is that FastMail is based in Australia, which I believe has some backwards laws with respect to Internet privacy.