How to use independently loaded and installed ARM compiler in PlatformIO?

Exactly – that’s a subtetly when linking with archive files created from library files. The default is that PlatformIO will link libraries as archives, not with direct object files, and thus stuff like overwriting _weak functions, especially the interrupt handlers, won’t work.

This setting is also part of pretty much all other existing FreeRTOS library, see e.g.

or as discussed in