Success!
I'm not entirely sure what actually happened, but the ROM chips work now. I'm a 100% sure in my previous attempts to have taken care about HI/LO.
However, in the meantime I fixed a faulty old Commodore RGB to VGA adapter - it had a faulty VGA connector that I replaced. This had been causing image to disappear, and may have coincided with my ROM chip switching attempts.
Thanks again for pointing to try again and double check!
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