Update Today: Laptop ran alright for two days, i shutdown normally and the next morning again laptop boots up to windows with black screen. I can see the BIOS ROG logo and as soon as it is about to go to login screen, I get black screen. Force shutdown, RTC reset by holding power button for 40 seconds doesn't work. I can only boot into windows to safe-mode with display though system repair process. DDU the display driver and boot normally into windows with Microsoft Basic Driver. When I try to install Nvidia Driver, during driver installation during second screen flicker I get black screen with driver install fail sound and black screen. Force shutdown and power up doesn't resolve. Safemode > DDU > windows boot works. However, doing DDU and Driver Install suddenly successfully installed the drivers with no black screen. After successful driver installation, I ran GPU-Z and all GPU information shows normal, I even ran Heaven Benchmark for half an hour and works fine. Then laptop runs normally for two days and same thing again.
Yesterday I found out the laptop doesn't POST using battery and fan spins fast with keyboard backlight on. Then when I connect the AC plug, it boots up normally after a coupe minutes with no POST. Also, If I remove AC while the laptop is running, I get black screen with windows running. I cleared CMOS, changed CMOS batter with a new one. This issue repeats. Battery charges and discharges fine btw, also battery health is good.
Note: ASUS Gaming Center gives me a Falied Overclocking message, saying BIOS>Defaul> Save, which I did. Even though I clearly didn't overclocked anything. (ASUS ROG GX800VH does overclock on its own when docked but even without docked I get Overclock Failed error message.)
I want to take it to a repair shop but I am scared that they would try to repaste ICs without care and nuclear the board. It's Bangladesh after all.

Any suggestion would be appreciated. I don't understand I'm not getting help from any forum sadly.