Welcome to the ROG forum !
Are you adjusting the brightness from the displays only?
If you have an Nvidia graphics card, open Nvidia Control Panel, under Display, select "adjust desktop color settings", here you can adjust the brightness and contrast as well.
Does this help you at all?
So the trouble is, your monitors only adjust the brightness up to 50% which leads to your PS5 games being too dark?
Doing a little search, I see many saying how their games look dark on the PS5, it seems turning off HDR makes games brighter.
If you have HDR enabled in the PS5 settings, try disabling HDR and see if your games are any brighter.