Why would you want to use the XTensa ESP8266 version for linking with a nordic-nrf52 type Cortex-M chip?
If you declare all your libraries via lib_deps in the platformio.ini, it should be able to be stand-alone and not need any Arduino IDE installation.
- You’re using
board = wiscore_rak4631. Where did you get the JSON board definition from? - Does adding
build_flags = -L".pio/libdeps/wiscore_rak4631/BSEC Software Library/src/cortex-m4/fpv4-sp-d16-hard"
to the platformio.ini make a difference? If not, post error message of a build (you previously showed a “Clean”)