Quick update for others that might run into this issue:
The F405US board that @dkemxr suggested above seems to be the correct board for the Mamba Stack Basic F405 Mini MK3.5 Stack 40A 6S 8Bit as I got the radio configured, the accelerometer configured and even the compass in the Beitian BN-880 GPS & Compass that I have was working, after connecting the battery to the quad. It seems that many of the sensors do not have power without the battery.
Unfortunately, plugging in the batter may also have been the reason for me frying the motors (and possibly the ESC) during one of the soft-reboots that were required by QGroundControl 
I would be really glad for any comments around this. Does a soft-reboot with a connected battery really fry things? (I have asked for comments on this in their section of the forum)