Sadly, it ended up being an APU issue. I’ve replaced the RAM chips and it didn’t make a difference. Then I reflowed the APU and it made it work, but the console immediately froze mid-game and “broke” again - so this was just an APU reaction to the heating process.
I’d recommend putting pressure on the back of the APU to see if that makes a difference, and running a memory test as I did. You could try heating the die with a hot air gun to simulate the heating process from a BGA machine and see if you get the same result as I did - then it’s probably the same issue. Fingers crossed that’s not the case 
As for the console, I reused all the parts and put in a new motherboard that I purchased faulty. It had a pulsing BLOD and I fixed it with the RAM chips from this console.