@Marc Saw that you have sold the player, I have the same player streaming from a windows SMB, had the long handshake issue. I have changed the deep color option to auto instead of 12 or 10bit, that makes the audio handshake before the video for me. Only able to confirm for 1080p files, don't have a 4k monitor.
My setup
Zappiti -> Marants sr7012 -> Darbee -> Epson TW9200.