35lbs is huge and going to single speed will not take 17lbs off your bike. I'm guessing 2+ lbs or so. Maybe a pound, or less, for the cassette, a pound for the derailleurs, and a little for lighter brakes levers/shift cables without shifters. Only issue is if you have hydraulic disc as a don't know if non shifter hydraulic brake levers exist.

I'm also having a tough time imagining you not having to get out of the saddle with a single speed. Not sure how that works with your back.

John