SOLVED. Controlling LCD voltage directly works like a charm. Thanks Felix :-)

I think I've Tried Everything:
a) M5.SetBrightness is tied to an ESP pin. No good
b) M5.Axp.ScreenBreath(x), whatever that is for, is not it. It also ends on an esp pin
c) m5.Axp.SetLcdVoltage(vvvv) and M5.Axp.SetDCVoltage(2,vvvv); don't do anything.
d) M5.Axp.SetDCDC3 won't turn the backlight on/off.

This is all I could find and try. M5 tough uses AXP192 but doesn't have anything connected to DCDD3. My M5Tough_Master library is not for this screen it seems. I'm out of magic.
Any help would be appreciated by me and the battery..