Yes, for me it was the y axis bolt adjustment. As said it’s stillt not perfect, but it seems much better! I’ll try to adjust some more and check if the y gantry itself is perfectly straight (but for that i have to disassemble some more…), and I also noticed that the x gantry seems to be skewed which I still have to fix.
This is my current mesh. As you can see it’s much better than the original one (but I also replaced the PEI sheet with a glass bed).
Honestly, I sort of lost touch with the printer. Now it’s not a tool anymore, but a project in itself. If I was in your shoes I would buy something other, maybe a Prusa i3? But I guess it’s also a question how much you are willing to pay…