And also for parts:
Fiberfix spokes, so you don't need 4 different lengths and so you can install it without removing your cassette.
Small pliers (I use a Leatherman Squirt PS4), used for many things, like turning a valve(core) or pulling a cable tight. These also have scissors which I want for non bike use.
Derailleur hanger for your frame
As far as the other items you asked about:
Pump: mid sized, to balance pmuping effiency and weight. Flexible hose to prevent ripping off a valve stem. Gauge so you know when you can stop pumping.
I use the Lezyne Alloy Drive, with replacement gauge hose, it works well for me a in serious comparison tests.
Go with flat pedals. With modern flat pedals you won't need any sort of strap of cage.
Make sure they have a 6mm hex head in the axle to install them, since you won't be carrying a pedal wrench or 10mm hex with you!
Traditional lycra shorts are the most comfortable, get some good ones. Then if it's cold, wet or you go into civilization, you can add something over the top.
Combo shorts have lousy cheap pads, and they are not multi-use. Great for a quick mtb ride or commuting, not for bikepacking.
All helmets are safe, the price difference reflects weight and venting. Get the best helmet you can afford. All that weight is up on top bouncing around with only your neck muscles holding it up, lighter is a lot nicer. Same for venting. A helmet is the one piece of bike gear you will wear all the time, every ride, so it's one of the best places to upgrade for more comfort.
You can find my gear list from my last trip here .