The issue with boot keyboard is exactly the same as outlined in HID Keyboard Packets
Boot keyboard needs to be a single keyboard device instead of a composite HID (keyboard, mouse, digitizer in our case) with multiple report ids. The patch from that post should give a basic idea on how to do that, no need to apply it directly, the changes can be done manually. If something still doesn’t work, there might be some issue in the HID descriptor in that patch specifically, however you might look for an example of boot keyboard HID descriptor and just use it instead (make sure to adjust the size as well).