Thanks for sticking through this and helping me out.
I too found out earlier today that perturbing the model via nodes offset/move might be a better way of introducing imperfection rather than an end moment.
I recently discovered that convenient API to move nodes by deformation.
I can feel a resurgence of energy and try to move on to more advanced NL buckling i.e. plates and ultimately stiffened panels.
Appreciate your help.