No idea why the ESP32-C3 needed to be written starting at 0x0, rather than 0x1000, but sure enough it did. I couldn't figure out why nothing could seem to be written successfully to my STAMP-C3, but basically nothing was working. Now, thanks to a blog post I found, it's successfully running the generic ESP32-C3 MicroPython binary.
Successful esptool.py commands:
esptool.py -p COMx -b 1500000 --before default_reset erase_flash
esptool.py -p COMx -b 1500000 --before default_reset write_flash -z 0x0 esp32c3-20210902-v1.17.bin
Result on REPL:
Build:Feb 7 2021
rst:0x1 (POWERON),boot:0xc (SPI_FAST_FLASH_BOOT)
mode:DIO, clock div:1
MicroPython v1.17 on 2021-09-02; ESP32C3 module with ESP32C3
Type "help()" for more information.