Hm interesting, we had a discussion about the Octopus board (Nucleo_f446ze does not support Arduino framework. Looking for advice) some time ago, but that was using a STM32F446ZE.
There’s also the related topic STM32F103RE Bootloader offset which makes a minimal example of a board based on the Marlin variant.
Can you try GitHub - maxgerhardt/pio-octopus-1.0.1-minimal?