I am a member like you and no snapmaker staff.
I am not so exerienced with cnc also. Never did a toolchange while in a cnc process, i carved it in multiple gcodes.
I would suggest to request this feature by email to support, you should also request M600 for 3d printing which maybe could your solving command in cnc also (it simply does the same, if its true).
One more have a look at this and maybe you are able to find here further information or a new postprocessor 
Sorry i see you did your way there, so its just for pointing out.
Maybe @Tracy could pick the request of cnc-toolchange up?