Bought some very cheap second hand #eink tags. Based on other people's observation the display is GDEH0213Z98. The #MCU on board is Unicmicro's UM3213, and fortunately the SDK and datasheet can be found on the company's website.

The SDK only supports Keil MDK. I made Gemini write a startup.s and a linker script for GCC based on the MDK files. Then I wrote a #PyOCD target (since they only provide a .FLM flash algorithm), some Makefiles and it works!

https://github.com/chuangzhu/um3213-sdk-gcc/tree/epd/Driver%26Example/gpio/demo