Doctors will prescribe a method that they find is effective, but IME, most of them really don't care what you do as long as you come in with the bowel condition that that want. If you know of a method that you can deal with that will work as needed and that you will actually follow properly, then ask to do that.
My doc prescribed one of the gallon liquid methods, but when I tried to drink it I could not keep it down; the amount of salt in it made me vomit instantly. I was having the procedure on a Monday and could not reach the doc, so I just looked up methods suggested by leading hospitals, and decided to go with the one I thought I could best tolerate. That turned out to be the Gatorade and Miralax method from the University of Michigan Med Center. it takes a substantial amount of Miralax powder and several quarts of pale Gatorade, plus a couple of laxative tablets. I don't like swallowing Miralax powder, so I mixed 1/2 cup of it with 12 ounces of a pale juice drink, mixed and chugged that down, and drank the Gatorade on ice slowly in between doses. My system was fully cleared out when I went to bed late that evening, so I got a decent night's sleep with no worry at that point. Doc was surprised when I told him afterward that I had had to use an alternative prep; he said he did not notice a difference from what he had expected. He told me that getting a clean result is more important than which method is used, and that any method that isn't fully followed is a bad choice because getting the job half-done screws up their ability to properly perform the test.
https://www.med.umich.edu/1libr/MPU/UMHS_Colonoscopy_Miralax_Gatorade_Prep.pdf