Maybe I am just not understanding what I am seeing.
What I think I see is a bundle of 5 wires soldered together to make 1 wire. That is all heat shrink together and then one wire goes to the power supply lug. If that's what I am seeing that is wrong regardless of what the prior owner did. Bundling them all up defeats the purpose of having multiple wires in the first place.
What you should have is 5 individual wires or at least 2 or 3 bundles with an individual wire going to the lug at the power supply.
And you do still have a voltage problem. 5.2v is too high and could/will cause damage to components. You are better off with slightly below 5 than above 5.1/5.2
I would make that one change to your harness at the power supply and then dial it back so you're getting 5.1 at the roms on the top board which will give you slightly below 5.0 on the bottom board.