You’re going to have to be more interactive and multimedia in order to solve this one. Photos of the screen for example.
Did you read and understand the setup instructions over at the Sony Megatron Colour Video Monitor Thread?
List your TV settings when in HDR Game Mode.
When I used my OLED, my OLED Light was maxed out.
Power Saving Mode should also be off or set to minimum.
From there, you should be using the Vulkan video driver.
You can also use D3DXX but for that to work, you also need HDR Enabled in Windows.
Load up RetroArch. Then Load your favourite content. Then load an HDR Preset. Verify if HDR is enabled in Settings–>Video–>HDR.
If you’re using a Sony Megatron Preset then you need to go into Quick Menu–>Shaders–>Shader Parameters then scroll down to the Peak Luminance and Paper white Luminance Settings.
Enter you display’s Peak Luminance. You can get that from RTINGS reviews or just leave it as it is. Or try something like maybe 800 or 1,000.
Then turn up your Paper White Luminance until the image gets bright enough.
Don’t mind the number, pay attention to how it looks.
What are your current Peak Luminance and Paper White Luminance values?
Be sure you update your Shaders using the Online Updater in order to have the latest Sony Megatron Colour Video Monitor, which is supposed to be v5.7.
It is also very important to have the correct version of the CRT-Guest-Advanced-NTSC Shader installed in the correct folder.
The instructions for obtaining and installing those are right below the download links for the preset pack and also in the readme.txt file. More recent preset pack versions have these included.
All of this info can be found in the first post at the top of this thread.
Be sure to experiment with your TVs tonemapping settings. This could be darkening or overbrightening the image so you might want to see how things look with it on or off.
Also on the TV I used when making most of my Sony Megatron Presets HDR Bright was the brightest HDR Mode but I used HDR Game Mode. In HDR Bright mode I had to set my Paper White Luminance much lower than in HDR Game Mode where I had to really crank it up.
Hope this helps and I hope you can follow up with some experimentation of your own to see if you can get things functioning well on newer LG OLED TVs because they look and work great on the “old” ones.
By the way, make sure you have BFI disabled. Get everything working properly first before experimenting with that as that can kill brightness.