I am searching for something to sync my phone contacts between multiple phones for some time. Best case it shouldn't use DAVx^5 and should support Android and iOS. Thanks for your suggestions!
What I found so far:
Nextcloud - but needs DAVx^5
Radicale - also needs DAVx^5
SoGo - needs some CARD-DAV application
Doesn't SoGo use Activesync? This wouldn't need an app on your phone.
I tried the same setup as you a few years ago. It worked, but I didn't like SoGo for email so moved to Nextcloud/SnappyMail and DAVx/K9 on my phone. Have been using this fine since.