@Harry500 the way you solve the AVR firmware issue is maybe by cheating the system.
Connect the CE device directly to the TV. After booting you should have much more resolutions in kodi video settings. There should now be all resolutions the TV do really support.
Open a SSH connection while the system is running and connected to the TV and dump the TV resolutions by running these commands:
cat /sys/class/amhdmitx/amhdmitx0/disp_cap > /storage/.kodi/userdata/disp_cap
sync
That’s it. Now power off everything, connect back the AVR like before and you should still have all resolutions in Kodi settings and they should work as well. Also you might get than “real” 4k instead downscale/upscale 4k.
About the EDID you can search the net about his purpose. I posted your EDID analyses above. The AVR normally read the EDID from the TV, modify it about the audio part and forward it to his clients. And here the AVR fuck up the EDID completely as only 720p is available → AVR firmware. You can try to contact the vendor but I am sure you will get no support.