We had this idea also. Actually I’m using a mix of @haydibe toolchain and a local folder.
The main issue with the extensions is the testing more than the compilation.
The 918 extensions need to be built with the syno toolkit and the tool chain for the modules to load correctly during the boot process otherwise the modules fail to load with “Unknown rela relocation: 4” message and you enter an install loop.
On top of that, the modules that fail to load during boot, load without any issue when I tested manually to load and unload on another system using insmod/rmm