I doubt the sort of jump you are looking to do will be painless, but I've never been physically dependent, and don't understand the neurology of such a situation well enough to say for sure. You will probably withdraw a lot going from 5g to 1.5g. As for the phenibut, I think its actually less potent, but f-phenibut would be (anecdotally) more potent, and maybe easier to taper.
If i were in your situation, id taper with f-phenibut, and maybe use some benzos towards the end of it. Sorry to hear man, that really sucks. Best wishes