I just wanted to chip in on the 4K issue. I've taken a look at getting a fully working device tree for LibreElec before, and I can confirm that the I2C communication on HDMI is broken when I checked last september. I wanted to get CEC working, but it seems that with FriendlyElec's default settings the wrong clocks are set. This prevents the T4/M4 reading the screens EDID and therefore only showing 'safe' frequencies and resolutions.
Now that the 5.x device tree files are in mainline Linux, maybe I can see what they changed in this regard and backport it to the current LibreElec builds.
