The master branch, to be released in version 2.1.0, from the Arduino Core STM32 platform adds support for the STM32WL55 series of microcontrollers. Extending the support to platformio's arduino framework would be great. I already managed to modify the board definitions locally in git forks and get the code to compile after fixing this issue: stm32duino/Arduino_Core_STM32#1473 as well, still need to submit the corresponding PR). I can make a pr once the Arduino Core STM32 issue has been resolved.