Back again. Just finished installing that Sourceforge .iso file for Rock Plasma 6. Then I ran the System Update. Did a reboot. No issues.
Next I installed task-lxqt from the terminal. Reboot again. No issues. I was able to get into both Plasma 6 and LXQt desktop environments.
A few months ago I had a situation, same ASUS, different SSD (sitting here off to the side) when I would boot and all I could do was get into the Failsafe X Session from the SDDM login screen. There were a few Plasma 5 files that got in with the Plasma 6 environment. Not sure if this solution would resolve your black screen after logging in.