Is there any way to output 4K @ SDR/BT2020? This would be a great alternative setting for some projectors that support WCG but don’t necessarily do HDR well.
Which bit of BT2020 do you want to eliminate? If you want 4K, BT2020 colours but a BT709 EOTF, you are asking for banding problems even if it was possible - which it isn’t, really.
Or if you mean output 4K as 1080p with BT2020 colours and EOTF, just use whitelisting and don’t choose the 4K modes.
Edit: OK, from some Googling, SDR BT2020 does seem to be a thing but I’ve still no idea what it means technically. So your PJ doesn’t support ‘HDR’ or just doesn’t do it right? If we are to consider it, please post the output of cat /sys/class/amhdmitx/amhdmitx0/rawedid with Vero connected direct to the projector.