Update. all above activity was with X11. I just noticed that I can choose Wayland desktop instead of X11 at login on the lower left.
When I'm using Wayland the HDMI display is 100% stable. It was flickering when I was on the login screen but after I chose Wayland and logged in, the issue was resolved. This is ok to close, but If you want to further triage X11 you can keep it open and let me know if you have any updates I can test. Thanks! ~S

Full 8nstall still flickers. {F10963376}

Hi Reilly -- I tested the live ISOs with a different display and cable at work and encountered the same problems on gnome and budgie, and it seemed less dramatically flickery on kde. All attached files are at first bootup.
One other thing I did was since my work screen is in portrait orientation I changed the display config in each to portrait for the extended display and the problem seemed to go away (I did this in gnome and kde and there was still some intermittent flickering on kde)