I would try using one of the probe methods that I mentioned in the other post. When using one of those probes you can have Marlin probe the bed in grid pattern and you select how many spots it probes. So if you could select nine probing spots and it will then probe the corners and five other spots on your bed. I don’t know what the max number of probing spots is but I know it’s more than nine. Auto bed leveling should be able to fix your issue even if it is just the x rods bowing in the middle. One of the main reasons people use auto bed leveling is they are unable to manually level their because of problems like you described.