>>2430653>>2430864>multi dayhow many days? that affects the answer.
buy your backpack as the absolute last thing, get all your other gear first. measure the volume of it using garbage bags of known volume. add a bit of extra space for food and wiggle room and because its hard to pack everything as efficiently in the field as at home. until you know this volume don't buy a backpack.
>what backpacks are goodif you go with milsurp looking backpacks, or traditional backpacking backpacks, both of those styles are going to have a mess of compartments and a ton of useless straps and other bullshit. you could just take a pair of scissors to one of those and cut all the useless bullshit off of it, but there's also backpacks that are just designed more minimally and actually for what you're doing.
i would suggest looking at backpacks that are a middle ground between ultralight and traditional backpacking packs. ones that have the design of ultralight packs, but with an actual frame and suspension, maybe a hip belt and a few amenities.
very cheap - Take a pair of scissors to the pack you have and try to make it decent
cheap - ULA CDT/OHM
expensive - Seek Outside
as examples there's many other options in this middle ground style. avoid dyneema composite, standard nylon ripstop or xpac are both good off trail.
both those come with custom sizing so that shouldn't be an issue. get a buddy to measure you.