[zvr]

It's pretty straightforward to sync the contacts on the phone with the contacts on Fastmail via CardDAV, using DAVx5 for example.

[x0x0]

I've been using fastmail for over a decade. davx is fragile. Many phone brands (all the OnePlus for example) interact poorly with it. I regularly have to trigger syncs by hand.

Still better than google, but the story here is not great.