I completely feel your pain on this one! It is incredibly frustrating when you spend good money on a premium bike and a top-tier Garmin unit, only to find out they refuse to play nicely together because of a "walled-garden" software approach.It definitely feels like a step backward when older or lesser-known systems manage to integrate just fine. Fingers crossed Bosch eventually pushes a firmware update to open up their ecosystem to standard Garmin/HR sensors, but like you said, I wouldn't hold my breath.

Hope you manage to find a decent .fit file stitcher in the meantime. Hang in there!