[KiwiCoder]

Not OP, but I moved all of my email from free Gmail to paid Fastmail about 2 years ago, and I've not had a moment of regret.

I send quite a lot of email for work and other things and I have what I think is a higher-than-average number of email addresses to manage.

One adjustment I made - I now archive older emails to avoid extra storage costs, which in turn means that searching for older emails is slightly more hassle.

Fastmail's web interface and mobile app are not quite as slick (as gmail) but this isn't a big deal for me personally.

[Sylos]

Can't you just use another e-mail app, like for example K-9 Mail, with it?

[KiwiCoder]

I used K-9 for many months and it was just OK. The FastMail app was not a huge improvement (over K-9) but it was still an improvement, at least in my experience. From memory K-9 didn't handle multiple "send-from" accounts as competently as the FastMail app, and there were other quirks that became more irritating over time.