I think I'm on the opposite end of your question but I'll drop in. Hello from DELCO by the way . . .
I spend most of my time small stream fishing. I'll seek out a brookie stream on an SGL and plan a day around an out-and-back that could be as long as 10 miles. My pack contains everything I need for the day including all contingencies I think I need: first aid, replacement BOAs, wader repair, bourbon, etc. You name it, I carry it in a pack that runs about 13-1/2 lbs fully loaded.
Look here for a little insight:
https://www.paflyfish.com/threads/my-small-stream-kit.82603/
I acknowledge that compared to what other people carry, I'm absolutely on the heavy side of what I bring but it works for me.
Now, when I hear "backpacking" like you ref it, I'm thinking you're counting fractions of an ounce, minimizing weight, and willing to pay a premium to shave an ounce off your pack weight.
If I'm tracking that right, I'd offer that you can get away carrying everything you need for a successful day of fly fishing while backpacking with what you would typically hang off a lanyard: nippers, foreceps, floatant, probably no more than 2 spools of tippet (I'd personally take 4x and 5x), 2 small boxes of flies - 1 wet; 1 dry, and a spare leader. Maybe one or two indicators. Maybe an Amadou patch. Maybe hang an empty third tippet spool with a mono rig wrapped around it.
That's it. From a threshold perspective and If you're counting ounces, I don't think there's anytihng else required . . . except of course the rod and reel you're spooling up.