Most flyfishing industry solutions are bloated. They perpetuate the need for excess.
Quick shots:
For short local trips. The quick shots where I drive up and the fishing duration might be under 3 hours. I will use the zimmerbuilt sling lite.
Unpacked it is lite as a feather, and will use the storage for carrying or sheding layers and maybe an energy bar. I will hydrate at the trailhead and leave water at the car.
Flybox, two rods, nippers, forcepts, fly line and tippet.
Optional:
Energy bar, bug repellent, ultra compact rain jacket.
Long shots:
Long hikes and full day trips.
I dont fuss with flyfishing packs. They are not great at hauling gear. I like osprey day packs which have great load distribution and pull the load away from your back and have a mesh vent to keep us cool.
Buy a volume to carry whatever junk you need to haul. I have one to haul my mess kit and backpacking stove and food to cook.
When I fish a stretch, i hang the pack in a tree and use the zimmerbuit strap pack modified for belt mount.
Flybox, nippers, forcepts, fly line and tippet.
On the water what else do we need?