Nice improvement! I think in my fast DIP system I got to about 35MHz but either the Dormann tests or my memory test started to fail in soak testing above that point. Similar to what you said here, that system also deliberately had very little on the data bus - just the 6502, RAM, and some transceivers in case non-RAM devices needed to be accessed. I'm not sure what exactly was failing above that point, it was hard to probe it and work out where the margins were too narrow.
Something nice about the 56Hz SVGA is that the horizontal total is 1024 - this was handy for me as I was using a RAM lookup for producing sync signals. It might also help with your MPU-driven system.
6502.org
The 6502 Microprocessor Resource