Hello JoergSH,
You can't directly use USB serial to BNO055 because BNO055 is 3.3V, so you need level shifter.
We already open our shuttle board schematic, UART interface and Driver as below.
https://www.bosch-sensortec.com/media/boschsensortec/downloads/shuttle_board_flyer/bst-dhw-fl039.pdf
https://www.bosch-sensortec.com/media/boschsensortec/downloads/application_notes_1/bst-bno055-an012.pdf
https://github.com/BoschSensortec/BNO055_driver
If you can design hardware properly, technically you can design your own hardware with BNO055 and test it.
However, there is always some possibility to have hardware issue.
Why not only bosch, but all chip vendors have their own development kit is for customer to verify their software first, then test with their own hardware.
Thanks,
... View more