Ditto this.

I found that out the hard way when I bought a day pack. First went to the spec busters, but teventually settled on Deuter Futura 32 which in theory was the heaviest in my shortlist, but ultimately the right choice as the fit was right and when loaded the best fittng backpack actually felt easier to carry.

I'd figure out and buy the contents you need first, get the lightest weight components that can serve the needs. Buy all that stuf first.

Then assemble those things and see what kind of weight and volume you're looking for. That leads you to a shortlist of backpacks that can do the load spec. From those, pick the most comfortable pack with features that you need, if there's a tie then go with the lighter one. .