OK, so for test I flashed Android 12. Actually, it took flashing Android 11 via EMMA, re-locking the bootloader and then spending several hours applying multiple OTA updates: 3 or 4 Android 11 updates and then 3-4 Android 12 updates up to the highest available Android 12 version (then an update to Android 13 was offered, which I cancelled).
The actual Android 12 version that I have is 62.1.A.0.675 (the latest available).
I’ve called several people and they said that there is no trace of echo, even with max volume level. The display doesn’t exhibit any trace of color banding or tint, either. So those problems seem to have been fixed already in (or before) that Android 12 update. I will keep using it, as in such case there’s apparently no need to use Android 13.
Actually, I haven’t noticed any differences compared to Android 13. Neither advantages, nor problems.
P.S. I’ve used SFOS restored from rootfs/home image made using dd.