@spockfish Wow. What a massive difference installing these RTL8812BU adaptors [Amazon.de, sorry] has made for my four RoPieeeXL installations. No more slightly asynchronous playback with the Devialet Phantoms and Reactors using AirPlay and Roon. I can recommend the update to anyone who currently relies on the built-in WiFi of the RPi 3b+. All adapters are running on 802.11ac with reported 866 Mbps and signal strength between -51dBm and -64dBm. This is up from a reported 150 Mbps and signal strength between -70dBm and around -65dBm. Why didn’t I do this earlier? 
Quick and easy process for upgrading.
This assumes that the RPi are already running on internal WiFi:
- Connect the new USB WiFi adaptor to the running RPi (any USB port will do)
- Go to the web interface, network and hit “Commit Changes”
- Go to your router interface and check for the IP of the reconfigured RPi. Because the RPi is using new hardware at that point, the IP will change as the devices MAC address will be different.
- Enter the IP into your browser address bar to go to the RoPieee control interface, now already using the external adaptor for WiFi.
- Go to the Network tab, hit “More Info” in the Wireless section, confirm that the external interface (the adaptor) is being used, hit “Reboot” in the alert box at the top.
At this point the RPi should be up and running with the newly installed adapter. Hope this helps someone.