Update: Decided to give "Legacy Option Roms"
Enabled another shot. Graphics set to "
Auto" in BIOS. RX570 is connected to DELL-2408WFP monitor via DP->DP cable. With this option, at least the hack goes to Sleep and Wakes. Only issue is that the monitor shows only a tiny part of the screen and flashes. Mouse-cursor is visible and can move.
Video Clip:
I plugged in another monitor through HDMI->HDMI cable on one of the HDMI ports on the RX570. Surprisingly the screen on the HDMI monitor showed my desktop. See attached pic. The first DP monitor continues flashing though.
Video:
Disconnected the DP cable from the GPU. Now, the HDMI monitor is perfect. Sleep-Wake works perfectly from now-on, with the HDMI monitor alone. I could get similar behavior with a single monitor that has both DP and HDMI inputs.
So, I hoped to go down HDMI only option. But unfortunately this didn't work. When I boot with just the HDMI connected, Clover screen shows funky Green or Pink color background, but Apple logo shows. Then after the GPU gets initialized, the HDMI screen never comes on. It constantly keeps going in and out of Power-save mode with no video. If I can get Clover to boot with just HDMI then we will have a perfectly workable solution. Did a whole bunch of googling and trawling the posts, but no clear suggestions on how to make an HDMI connected monitor to work with Clover. I can upload the problem reporting files if it would help.
I am hoping debugging/fixing this might be easier than dealing with the hard crash that results with 'Legacy Option Roms" disabled.
A minor issue: my HDMI monitor wanted to use YPbPr mode for some reason. Had to edit the EDID to correct it. My Optimac looked glorious