The weight of the coupler is negligible
This will be a low power rocket. Static tests are typically done with high power rockets. The ejection charge is separate from the motor. Only the charge is loaded.
This rocket will take a year to complete but I can get it to the testing phase in a few months. I'll be using a single-use Estes black powder motor with a max thrust of 7.4 lbs. The ejection charge is part of the motor. I've never seen a live-motor ground test done with a model rocket. It'll be interesting.