Looks very similar to Atari Max and Williams Bank Switching Schemes. Any way it can use XEGS bank switching? $A000 to $BFFF always mapped into memory, $8000 to $8FFF changing. Can either be writing to $D500 only or $D500 to $D51F. I like the XEGS because you have a section that never changes for your "Top Logic" and VBI and use the other banks for Data and callable routines. Can you plug in the XEGS control logic into your cart?   How is the quality on these cartridge boards if they are plugge