I like native apps instead of having everything in the browser and while Apple Mail gets along fine with fastmail, I do prefer native Fastmail so I can quickly create rules if needed.
This is a neat app written in swift that I found which is free and also runs natively on m1.
Is it not available in the Mac App Store?
I use this open source alternative:
It is not in the Mac App Store. Lots of good Mac apps aren’t because it’s such a hassle to deal with Apple’s restrictions.
FMail is my preferred way to use Fastmail. The developer is very responsive.