I have had very little time to put into the project, but it is still my highest priority personal project.  I had to respin both the cart and the programmer board because of errors.  But I do have the cart able to store and readback a simple program.  Since my gigatron is not correctly running code yet, I loaded the first program the original gigatron ran into the flash cartridge - just simple loads and a jump as recommended by Marcel.  I have just finished verifying it on the cart, and need to analyze it running on the gigatron.

https://hackaday.io/project/20781-gigatron-ttl-microcomputer/log/58830-first-program-run

I also found a few schematic errors because I did not copy the gigatron project exactly.  I fixed those on the gigatron by cutting traces and soldering jumper wires, but that did not immediately solve my problems.  So next up is functional testing with the custom code.