And it works!That's very strange - no response from the !CS line at all, not even to try to initialize the SD card? My first thought is to try swapping the PICs from one to the other - if the failure follows the board, maybe there's an issue with power; if the failure follows the PIC, then... maybe ESD? It'd be very strange if that were the case, but it's not impossible.
I was beginning to second guess my soldering, but finding an issue would be too much of an ego blow to a dedicated vintage hobbyist!
The issue was I hadn't jumpered J3 on the Demik TashTwenty. Jumpering J3 allowed both my Transcend microSD card and the regular size Transcend SD card to boot my Demik TT.
So to summarize, both my TT2.0 and DTT work great with the Transcend cards I bought on Amazon. I was unsuccessful getting my Sandisk or other brands to work (error code 0x05) but given that changing card brands worked, I'll stick with the current PIC firmware.
Thanks again and hopefully my experience helps others.