Espruino on your watch!

The ID205 is also listed here https://idoosmart.manufacturer.globalsources.com/si/6008849563328/pdtl/Smart-watch/1165759500/smart-watch.htm which I suppose is the page of the "Shenzhen DO Intelligent Technology Co. Ltd" manufacturer. The page mentions "Very Fit Pro" app and "Main CPU: Nordic nRF52842". If you are interested in direct veryfit app hosted firmware download link let me know. It is signed firmware, the .bin file inside is ~700K and initial stack pointer points to 0x2003F3F8 so SRAM is 256KB, interrupt vectors point to 0x26xxx so 0x26000 is app start which implies soft device version. Also "nrfutil pkg display" shows info like:

  |- Type: application
   |- Image file: ID205.bin
   |- Init packet file: ID205.dat
      |- signature_type: ECDSA_P256_SHA256
      |- hw_version 0x000000E2 (226)
      |- sd_req: 0xA9
      |- hash_type: SHA256

However I did not find this model here https://fccid.io/2AHFT which otherwise has most of their devices.

This was one of the first firmware files so I hoped there could be more nRF52840 ones but this was the only one after all. However good news is that their older nrf52 models are based on SDK11 so the firmware is not signed, here are model names (from FW metadata)

• ID130Plus Color HR - possibly https://fccid.io/2AHFT616/Users-Manual/Users-Manual-4126121
  
• ID132Color (bt name SAT 1500 B1)- https://idoosmart.manufacturer.globalsources.com/si/6008849563328/pdtl/Smart-bracelet/1162975468/Smart-bracelets.htm
  
• ID153
  
• ID151
  
• ID128_Color_HM - possibly https://fccid.io/2AHFT624/Users-Manual/Users-Manual-3965560
  
  

I also went throught WearFit app links but all nrf52 ones are signed (M2S,M6,eTimerFit,AISIRER-8C_HannStar) however there are lot of nrf51 based, all of them using the 32K SRAM variants so they are moderately usable too (K1,B15S,C1,CK11,E26,M8,OSSINE X7, P2 HR,SWB200,T-FLY,YKY_SH,Tempo,Z18).