I'm on the fence of liking the idea of a chopped full size glock, but if I did go that route I'd probably have it done by someone who has done it successfully several times, aka a professional.
Maybe you could talk to someone who does this as a service and get there opinion.
Also, have you thought about it could be the mag spring in your 20 mag? Is it the same G20 mag every time? Could be the spring(s) getting weak.