Acer laptop randomly won't boot

Hi, I have Acer Aspire E5-576G that randomly won’t boot. There’s no certain pattern as to why this is happening, but sometimes it happens when I shut down the laptop or let it go into sleep. After that, it will not boot whatsoever.

I noticed that there are two scenarios it will act upon pressing the power button:

1. Power LED is on but no fan spin, no HDD, blank screen, and the laptop shuts down after 3-5 seconds
2. The power LED is on, the fan is spinning for 1 sec, HDD spins up but stops after 2sec, the screen shows Acer logo and goes blank, eventually shutting down the laptop in 3-5sec.

Things I’ve done so far:

1. Tried booting into BIOS - even though I was able to go to the bios, I couldn’t do anything there as the laptop will shut down in seconds.
2. Tried using a different power cable - the same thing
3. Disassembled the laptop completely and disconnected every possible component from the motherboard (except RAM, power cable, and fans) - acts according to scenarios 1 and 2
4. Tried using different RAM sticks (that work on another laptop), both dual and single-channel - no luck here as well.
5. Disconnected the battery, power cable and removed that little BIOS battery on the motherboard, then held the power button for 60 seconds, after that put everything back together - no boot
6. While doing those tests the laptop appeared to boot up once or twice (showing no bootable device logo as I had disconnected the HDD), but I turned it off and on again to make sure it was working properly - no boot after that.

And the most annoying thing is that the next morning it will boot up as nothing happened, and who knows when it will not boot up again…

I’ve never seen anything like that before, so I’m kinda lost on this one…