Messages and FaceTime are both working on my ASRock X570 Taichi with Ryzen 9 3900X. Installed macOS 10.14.6 this past week. Here's how:
Because I have both Windows and macOS installed, I use the built-in AX200 WiFi/BT in Windows. But for macOS, I purchased and installed a Fenvi FV-T919 Broadcom card. This is natively supported in macOS, so no kexts of any kind are needed.
However, to prevent the built-in AX200 Bluetooth from interfering with the Fenvi card, we need to disable the PRT2 USB port on XHC0. The attached SSDT can be coped to your CLOVER/ACPI/patched folder. Then reboot. This will only disable the AX200 in macOS.
Also, if you're using an NVMe SSD for macOS and it is appearing on the desktop with an Orange icon (i.e. as an external drive), I have an SSDT that can fix that as well. However, to adapt this SSDT for your system, just do this:
1. Download and run IORegistryExplorer (search Google for link)
2. Scroll the device tree on left side until you see your NVMe. It will be under GPP0 or GPP1 or GPP2...
3. Let me know which GPP(x) it is attached to.