Joy! I got it up and running on an ESP-WROOM-32 (Board: ESP32-WROOM-DA Module) in GxEPD2
Never thought I'd be so happy to see "Hello World!"
FTR, you were right, the "no inking" driver is what finally worked. I want to say I did try that on the S3 but I probably had something else wrong at the time that I missed. Going to try switching back to the S3 now, since this is clearly working.
You made my day 
Thanks!
Oh, and I also had to use the B/W panel for it to work:
GxEPD2_BW<GxEPD2_420_GDEY042T81, GxEPD2_420_GDEY042T81::HEIGHT> display(GxEPD2_420_GDEY042T81(/*CS=*/ 26, /*DC=*/ 25, /*RST=*/ 33, /*BUSY=*/ 32)); // GDEY042T81, 400x300, SSD1683 (no inking)
...
#define GxEPD2_DRIVER_CLASS GxEPD2_420c_GDEY0420F51 // GDEY0420F51 400x300, HX8717 (no inking)
As I was trying to figure out how to get it's two grays working, I ran across the gray scale library that does work with this panel:
https://github.com/ZinggJM/GxEPD2_4G
So for that:
GxEPD2_4G_4G<GxEPD2_420_GDEY042T81, GxEPD2_420_GDEY042T81::HEIGHT> display(GxEPD2_420_GDEY042T81(/*CS=5*/ 26, /*DC=*/ 25, /*RST=*/ 33, /*BUSY=*/ 32)); // GDEY042T81, 400x300, SSD1683 (no inking)
...
#define GxEPD2_DRIVER_CLASS GxEPD2_420_GDEY042T81 // GDEY042T81 400x300, SSD1683 (no inking)