Unless you plan on taking your bag on long hikes, I recommend clamshell style backpacks. I have a 40L one that I have taken around the world and I tend to overpack. It is way easier to organize/pack. I also used a 55L amazon basic hiking bag and that worked perfectly fine. It can get a bit clunky and you might have a harder time sneaking it past discount airlines with no free carryon restrictions.
Things that have been incredibly useful to me
compressible bubble down jacket (like the patagonia micropuff but I got the Amazon basic one).
Convertible pants. You now can pack less pants/shorts.
Merino Wool sweater. Slightly pricey but great for layering. You'll be surprised how cold it can get at altitude.
Rain poncho/tarp. They are all over amazon and they fold down pretty small. You can use them in a ton of different situations and I have found them to be more useful than a raincoat (which is what I use to pack).
flashlight. Get those rechargeable ones they recommend on /r/flashlight.
a good battery pack for when you don't have easy access to electricity. It should be light/small enough you don't mind carrying it in your day back but large enough for at least 2 charges (I carry a 10kmah one).
if you are worried about weight/packing I'd advise investing in quick drying shirts/pants/clothing. You can easily hand wash your clothes and it will be dry in hours in most situations. 3-4 shirts, 3 pairs of pants, and extra underwear/socks should be about all you'll need.
carabiners and paracords are your best friend. They are incredibly versatile and useful. You can easily attach things to your bag or use the paracord as a clothe line.
$19 without the battery (they are sold out of the 18650 battery) on the wurkkos site.
$26.39 with a battery on Amazon:
The AceBeam Pokelit is $17 and up:
https://www.amazon.com/ACEBEAM-Pokelit-AA-Rechargeable-Flashlight/dp/B0B1Q646BK/
It has USB-C charging on its internal battery so it should be quite waterproof.
The Wurkkos FC11 is $27:
https://www.amazon.com/Wurkkos-Flashlight-1300Lumen-Waterproof-Activities/dp/B08JCM95X6/
It is rated IP67 (submersion waterproof) as long as the USB-C charging cover is in place properly.
The Streamlight Microstream USB is $33:
https://www.amazon.com/Streamlight-66604-Microstream-Black-Flashlight/dp/B077BLB1DN/
It has a USB charging port behind an o-ring so it should be more reliable than a flap. The last one I held was micro-USB, they might have been upgraded to USB-C (I can't tell from the Amazon page).
The FC11 is the largest, but it will be brighter and has a larger battery.