Great job, that is going to be a "mopar Cathedral", a place to give thanks for the e-gods,lol.
About the finishing part for the mud work before painting, unless the mud is really put on extra heavy you can use a damp sponge to smooth out the seams and screw dimples, just keep rinsing out the mud in a bucket, just be careful as to not get the open board areas too wet. I remoldeled our kitchen in the middle of winter and couldn't open any windows or doors for ventilation so as to get away from the dust and used this method, it works. The dry sanding dust part otherwise is a real mess.
