The bed news is that it's not possible to update softdevice/bootloader with standard weloop bootloader. When you try to update the firmware/application it works but when you try to update bootloader/softdevice it just restarts.
The good news is that we can write a custom application that will do this. Now I'm working on an app that will fetch new bootloader/softdevice using nordic UART over BLE and save it in external flash. When everything is uploaded then it will copy data from external flash to internal flash and we will have a new softdevice/bootloader. And I have to write it using nrf 5.2.0 SDK which is really annoying ;)
There was only one problem... I didn't know what external flash is in this watch. After some research I found out that there is a standard spi command that works with almost every flash chip. It's 90h and returns chip version but that version may be shared between many variants of the same chip. Then I found a newer command: 9Fh and now I'm 100% sure what chip is there.
Manufacturer Id: 0xEF
Device Id: 0x4013
So it's definitely THIS chip :)