A quick update after a long time.
I found the problem really was with the VIA (6522).
It is essential in the Mac startup process.
I replaced this chip and now have a working Mac 512 again.
So if you're seeing a similar pattern on your Mac 128/512, be wary of ROMs first and then 6522.
I have a Mac 512K that just displays that checkered pattern at boot.
I already replaced the ROMs with those of a working Plus.
In fact, even without any ROM, the pattern is the same.
The continuity of all ROM pins is fine and voltages are ok, too (recaped analog board).