I would honestly just go into any decent outdoors shop and try a few things on for comfort. For me, 40 litre is a good size, not saggy when it’s half full and they tend to come with a supportive hip belt.
Features I require:
Ski pockets and two straps each side for my mast and walking poles
Single main compartment
Lid pocket and inner lid pocket
Zip away rain cover is handy
Supportive waist belt and chest strap
Ice axe loops and crampon straps or webbing
If it has side pockets, they must be bellow type so they can fold flat to allow pole storage.
Back venting not an issue for me as I’m not really a sweater.
My current summer SOTA sack is a Vango 40l one which was a freebie with a subscription to a climbing mag and has had regular use over the past 6 years without any issues. It has all of the above features. (No side pockets though.)
My winter pack tends to be a Berghaus Extreme 40+10, from my winter climbing days. It’s about 18 years old. As well as the above features, it has a snow shovel pocket. It’s good for lightweight overnighters too, as it has a small bottom section why can take a sleeping bag, bivi-bag and mat. The interior divider can be opened when a single large space is needed.
I’ve also got Lowe Alpine and Marmot packs, which are also great. My son inherited my 30 year old Karrimor 40l pack, complete with its 1990’s pink and blue colour scheme. It’s still going strong.
I think some of the brands are way too expensive. Especially the likes of Osprey and Deuter, but that’s just my opinion.
Good luck hunting!
Fraser