First, yes you need a different Roku for HDR. The Roku 4 is 4K, but not HDR. As far as the 4:2:0 setting, it's no longer hidden in a secret menu, but is under System/Advanced System Settings. I don't have any experience with Onkyo, buy my Yamaha AVRs have an advanced menu that has a setting that has to be correct for full 4K/60 Hz. Yamaha labels it "Mode" and Mode 1 is necessary for full 4K/60 passthrough. . Personally, the biggest issue my Ultra ever had was when I replaced my 4 and I couldn't get a good (reliable) 4K resolution setting. For me the issue was my HDMI cables weren't sufficient for 4K/60. They have to be certified for 18 Gbps data speed. I bought a couple of Amazon Basics cables and haven't had an issue since. Every once in a while I see the HDCP error message, but it always disappears in less than a second. Not enough time to even read the warning.
Dan Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.