For email you can import your gmail account via IMAP to fastmail. I have a custom domain so it's been awesome to finally serve mail from it and a relatively painless transition.

The problem I've been having with this is that most of these alternative providers don't offer much space, even on their most expensive plan. My GMail storage use is around 300GB right now. Sure I could probably clean that up if I wanted to, but I don't. I'd rather just pay for 400GB of space, with options to go much higher as time goes on.

Fastmail's most expensive plan offers 100GB, and most alternative providers seem to max out in this range (or lower). Mailbox.org is the only thing that I've found that is (or at least claims to be) privacy-respecting that offers lots of space.

On a side note, given some of the anti-privacy and anti-encryption legislation passed in Australia in the last couple years, I'm not sure Fastmail would be my first choice to replace Google.

You can buy more storage at Fastmail: it's the same cost as an extra user to add the same amount of storage. e.g. for a Professional account, you can add an extra 100 GB storage for an extra $9/month.

Gmail reports a combined storage with Drive etc., so you may find not all of your quota use is actually email.

You can try Migadu, which is priced by number of outgoing emails per day and not by storage or domains or mailboxes. The company is in Switzerland, but it uses a data center in France. I personally have very limited experience with the service, and cannot give either a strong yes or no for it.

plenty. https://lmgtfy.com/?q=email+hosting+providers

fastmail.fm seems well regarded. (it's not on the first page of results above)