I've read you post several times but I'm still confused what does and doesn't work.
Code
I could borrow a Pi 4B 4GB, it is around the same age.
Same SD-Card, connected by HDMI to be HP Screen:
LibreElec runs on my Pi 4B 8 GBLibreElec runs on the Pi 4B 4 GB
So "my Pi 4B 8 GB" is the Pi you have been testing throughout this thread. Which I thought hung on splash screen.
Are you stating LibreElec runs (i.e. works normally) on this Pi?
Is everything the same in the tests of the two pi (as well as sdcard, the power supply, cables, display, peripherals)?
It sounds like you changed hdmi resolution (looks like to 1280x720 sbs @ 50Hz). Note that sbs is a 3d mode and probably shouldn't be used as default.
Is this on your original problematic pi, or the borrowed pi?