I have the same issue on my QN90A using a 6900XT. I have managed to get HDR in ST2084 with Freesync Premium engaged in 120HZ but either AMD driver is messed up or QN90A EDID is messed up, because Freesync Premium Pro is causing the TV gamma to lock at 2.2, not ST2084, causing the dim picture. NVidia 3080 I had previously used VRR and HDR perfectly. If only we could exchange freesync for hdmi forum vrr... Anyways do this to get brighter HDR...
Make sure tv input signal plus is enabled for hdmi port 4, which we will be using as this is the only port with hdmi 2.1 support. In TV menu it is settings>general>external device manager>input signal plus
In CRU folder, run reset all and then restart 64 so we are starting fresh. TV screen will go blank for a moment.
In tv menu, make sure your TV is in PC mode as the source. If it is not, unplug the hdmi and put back in port 4. It will ask if your device is a PC and you push the down arrow on the tv remote to confirm. Now turn game mode off. It should not change, but make sure you are still running HDR and 120HZ in windows display settings if it is not already, then 4:4:4 colour in AMD settings. Freesync will be disabled for now.
Open CRU and click on Extension Block (2 slots left) listing at bottom so it is highlighted blue and then click the edit button below.
Highlight TV resolutions and click edit. Delete the 24/25/30hz options here to free up space in the extension block and click ok.
Now, click add, and select freesync range from the menu. Put 48-120Hz. Click ok. Click ok again and you should be back to the cru main page. Click export and save your file as any name you want but leave format as .bin file
Now, close cru and put tv back to game mode and make sure you enable 120hz, 4k, hdr and full rgb 4:4:4 or ycbcr 4:4:4. It should be on freesync premium pro and looking dim as it was before. Open cru again. Click on import and load the file you just saved, leave CRU open. Now run restart64.exe in cru folder. Your display should now be detected again in freesync premium (not pro), which seperates the hdr from freesync, with the tv doing the processing using the st2084 gamma curve, resulting in a much brighter picture and better hdr all around. Close CRU.
Lastly, go in to your device manager and in the view tab, choose show hidden items. Click on the monitor listing and remove the listings for the qn90a which are not in use (greyed out) by right clicking and choosing uninstall. This will ensure it always detects the display in the mode you are in. I would also recommend going to the microsoft store and downloading Windows HDR Calibration. This will run you through a quick and easy calibration, similar to PS5 which will get your HDR perfect.
There is still an issue where switching between hdr on/off or changing resolution or refresh rate can cause signal loss. I have found that if I keep moving my mouse whilst making these changes, the signal is never lost. If you do lose signal, switch to another hdmi port and back again using tv remote. If still not detected, push ctrl, alt and delete together and try again as the screen which pops up by pushing these buttons is consistently detected for some reason...
I hope this helps you and others, please do not hesitate to reply and ask anything if you encounter any problems 
