Hello Djed..... Not to be a party pooper, but I believe that if everything else on your machine were perfect, that the router being out of square would produce the same pattern of markings at all four corners. These systems can get really confusing to figure out, and must admit I have had similar problems that I could never fully resolve.
I use a mechanical dial indicator, mounted on a trammel arm chucked into my spindle collet. And despite spending hours, using super thin shims, I could never get a full rotation of the trammel arm without variations in the dial indicator. (Off hand, I don't recall my readings, as this was a couple of years ago.)
Then, feeling that I had readings as good as I could get, I'd move the spindle to some other spot on the table, and re-check my trammel readings, only to find they were far different then just a couple of feet away. (Again, I'm being vague with numbers, as I don't recall any longer (though they are probably in one of my past posts.....)
It was recommended that I check my pinions for tightness and wear, and I replaced them. Things were still off.
Some suggested my spoil board was loose, and, well, as I noted, it can get very difficult to figure out what to do.
At least take notes on what you do, and what you discover, as it may help others. And, I would suggest that you populate your personal information fields, so people can tell where you are located. Maybe you have a super helpful, expert shopbot user close by, who could offer assistance......
Good luck,,,,,,,,,,,,, Chuck