I will say I have used the Oppo 103D since it came out (replaced a 93) and it is a great Bluray player - works flawlessly with the HD621. I also just bought the 203 and am experimenting to get the best UHD settings for my system (there are issues with UHD HDR and projectors) and I'm confident Oppo will get everything figured out with future firmware updates. Oppo is such a great company with terrific support ( reminds me of the old M as you can call them direct to get questions answered).

As far as the HD621 - I use the HDMI 2nd audio only output of the 203 into the HD621 using PCM and my 861 reports the audio on UHD discs the same as Bluray (8 ch - 96K).

Since the HD621 can't pass 4K I use a direct HDMI from the 203 to my projector for video and the only frustrating part for me is I can't leave a 1.4 HDMI cable attached from the HD621 pass through to the projector for other sources when I use the 203 as the audio will not play (I assume because the HD621 is looking for the video also).

So if I detach the pass through cable the HD621 will then play the audio from the Oppo 203.
I might add while I am waiting and hoping for a new version/product from M the 861v8 in 7.1 surround still sounds incredible even without the new audio codecs.

A HD621 replacement that handles HDMI 2.0 and above is also very much welcome and needed.

My gear: Trinnov CI, 271, 8000.1(SEs), 7200.1 HC (SE), 320s, 3200s, Seaton F18 subs, MS200 (2nd zone), 218, 210.
Other: JVC RS-4100 projector, Lumagen Pro Radiance, Panamorph DCR anamorphic lens, Stewart G4 130 screen, Kaleidescape Strato K, Rvolution 4K player and Zapitti NAS, Oppo BDP-203,Roon Nucleus