I use Carddav, caldav sync only if it support oneway sync from server to device.
I had already that often the problem that some apps change contact information and with two way sync all this changes will done also on the server without any real control. For example some apps like whatsup change the contact picture to an picture the user uploaded but this picture is often didn’t show the persons portrait. So I use a carddav and caldav server and change this information only through the webpage of this server. Then I regulary let a carddav, caldav sync tool download all information to my Android device. This way I will never get an destroid contact database.
May I ask you whether your sync tool will support one way sync as an option in the future? How does your tool sync contact pictures? Will they also synced in high quality?
thanks a lot best regards Wolfgang