So if you have changed the power supply and it’s still doing it it leads me to believe that you have a backlight problem and the only way to fix this is exactly what you are fearing get the brightest light possible or put the tv in a extremely well lit room so during start up it won’t call for the backlight to come on once it powers up go into your settings and turn it all the way down or off If none of this comes to a resolve then you most likely have burnt row of leds

Try pointing a bright light at your screen if it power’s on all the way turn off the back light,if not it’s your power supply try tapping on back of tv just replaced mine again after 5 years it’s important to keep the backs clear of debri so they run as cool as possible

Mine is the nsx-46gt1 Sony Google TV, replacing the power supply board fixed my tv red light flashing 6 times.