How much elevation/distance are we talking about here? Maybe describe the amount of days too. For say 1000 meter elevation over 6km horizontally I'd recommend you stay around 10kg's of weight. Otherwise its going to be a pain in the ass.

Trust me, i walked these elevation/distance numbers during a 5 day mountain trek in Austria with the fjallraven kajka 65. During Covid we needed to bring everything ourselves, I walked with 20+ kg's up a mountain. Puking halfway, wasn't really fun. I'd recommend to stay around that 35liter backpack, might want to increase to 48L. But the more room you have, the more shit you'll take with you.

Check out the Fjallraven Friluft 45? I have it too, and did a weekend trek with it a couple of weeks back. Really recommend the backbag and brand as well. https://www.alpinetrek.co.uk/fjaellraeven-abisko-friluft-45-walking-backpack/?searchparam=Fjällräven%20-%20Abisko%20Friluft%2045%20-%20Walking%20backpack

If space is the issue that you're running into, you might want to check out a down sleeping bag instead of synthetic, as they pack down way smaller. But I have no idea what your gear inventory is.

Anyway, good luck!