made an account to share a fix
firstly, would like to thank everyone here for contributing to solutions and being so helpful.
running a mac mini m4 in 2 cheap chinese screens, had the grid looking thing with occassional flickering on cheaper chinese screens, especially with darker images and backgrounds, downloaded betterdisplay and disabled gpu dithering (known issue with nvidia so tried it with mac) - improved a bit, changed color mode to 8-bit, RGB, fixed the problem for now (fingers crossed).
notes: did not have same issue with default settings with better screens (dell, acer etc), cables isolated from issue - tried diff cables with diff ports of diff quality.
chinese screens with windows did not replicate the issue that was with mac mini.
possible solution via software above, or use a different monitor.