a huge numbers of US soldiers use the Spec-Ops brand THE pack here:
http://www.specopsbrand.com/packs-bags/t-h-e-pack-tactical-packs.html
I know, I know. their marketing is pretty cheesy as well as the name but the product in itself is pretty solid. i got mine on active duty, used it including baghdad and fallujah, and still use it today as a daily civilian backpack (more than 10 years use). lots of soldiers carry this pack through various environments. it is the one of the more premium/pricey packs that is sold at the PX on military bases and is sold on a mass scale.
i think its a very solidly built pack, and its certainly well tested. it's 1000D cordura and 2550 cu. in = 40 liters, which is right in the middle of what you already have and what you're asking for.
if you need a bit bigger they have this newer one (of which i have no experience) that is 45 liters:
http://www.specopsbrand.com/packs-bags/t-h-e-pack-uap-ultimate-assault-pack.html
twice a year i spray mine down with 303 waterproof spray (
https://www.amazon.com/303-Fabric-U...rd_wg=EAhp4&psc=1&refRID=C67DEHG1BEXD0QQAZAHN) and it holds out rain/snow/slush just fine even when i commuted on a bicycle/moped/motorcycle.
i have a spec-ops pack among many other brands (timbuk2 messenger, reload messenger, filson briefcase, kifaru 29'r, multiple chrome bags, etc.) and i still opt to use my spec-ops daily. it just works and doesn't break the bank. i'll never understand the whole goruck movement except that they have clever marketing. kifaru packs are built reasonably well but are overpriced and their customer service/website is terrible, and also the reviews for their product on their website are censored, so i overall i wouldn't recommend.
i have heard some solid things about mystery ranch but no personal experience. these packs look pretty good but i don't know if they look like "backpacking" packs to you by your standard:
http://www.mysteryranch.com/Packs/Mountain/Climbing
also mission workshop backpacks are very bombproof, tend to be entirely waterproof, expandable in size (for roll-top bags) and don't look too tacticlol. if i was on the market for a new pack to replace my spec-ops i'd look at these:
https://missionworkshop.com/collections/backpacks
hope this helps.