Decathlon has an exceptionally good deal if you’re looking for a lightweight pack that won’t break the bank:
https://www.decathlon.co.uk/backpack-50l-20-khaki-id_8382535.html
I usually bikepack and use much smaller backpacks, but got this one for occasional hikes with my wife.
Mine weighs 960g (excluding the rain cover). It features a sewn-in plastic sheet that flexes a lot and seems marginally useless to me – so there’s more weight to be lost there, albeit with a bit of DIY effort.
Best,
Martin

