With 60,000 miles and all original parts, what should I start replacing? Should I upgrade any parts?
Keep driving it until something breaks and do the required maintenance.
Murphy’s Law: “If it is not broken don’t fix it”.
Maintain the truck at least as well as GM specifies in the Owner’s Manual.
If you want to change fluids and filters more often, that is even better.
When you drive it, try to make each drive long enough so that the engine, transmission, and exhaust system all warm up completely. Vehicles that have a lot of short trips do not last as long as vehicles that are consistently driven until they warm up fully.
And, be sure to wash the truck regularly and wax it with paste wax a couple of times per year. If you use it in the winter, be sure to have the undercarriage washed frequently to rid it of road salt.
Replacing parts prophylactically, as you are considering, is usually a bad idea, with some common exceptions being timing belts (if your truck has one), and air filters. As long as you are following the manufacturer’s maintenance schedule, which is usually in the owner’s manual, you should be fine. Avoid fluid flushes for all systems except the brake system. If your truck has two maintenance schedules, one for normal use, and one for extreme conditions, you might either follow the extreme conditions maintenance schedule or split the difference, depending on what kind of use the truck gets.