Just thought I'd share this while I'm putting it through its paces and attempting to make a break from Proton Mail: I'm using KashCal to sync Fastmail calendars to Android (well, Graphene), and so far they seem to play well together as well as with Betterbird.

What's nice about this app is that it has the functionality of DAVx5 built into it, and also it has some good calendar widgets. Not in any way associated with the programmer, just thought I'd pass this along.

EDIT: One thing I forgot to mention—when I first tried signing in to KashCal, it didn't accept my Fastmail password. Almost immediately afterward I got a message from Fastmail directing me to its web version where it asked me to authorize the KashCal API (?) and it generated a special password that Fastmail will only use with KashCal, for security/privacy reasons, just in case. Kudos to Fastmail for making that part so easy.