Power Options -> Change Plan Settings -> Change Advanced Power Settings -> Display:
Screen brightness: you choose the one you like (I have it at 80%)
Screen brightness attenuated: change it to 0% (in battery and in current)
Enable Adaptive Brightness: Off.
Now in BIOS:Advanced -> ThermalThermal ConfigurationIntel DPTF Charger Participant
-> Display Low Limit
: Here put the brightness that you want to keep as a minimum, as in the GPD Win I have it at 80%, because I leave it like that (before it was 40%).Intel Dynamic Power
(Optional): This also disable it (I think it is not necessary to disable it for the brightness theme, but well, if it does not work, with this, it works lol)