I use a similar setup on some of my outings. See link below.
I like belt mounts and like a loop comnection to maintain orientation when bushwhacking. A caribeaner mount would give more options and would be easier to relocate.
I really like your leather pack, once it is weathered I bet it will look really cool.
I use mine mostly on quick or long shot outings. Quick, as in not far from the car. Long shot, as in longer hikes where i might have a heavier pack that I will hang in a tree while I fish, and use the belt mount on the water.
In general, i think belt mount style is the best way to fish as it makes us more mobile. Slings and backpacks affect our posture.
I do use a sling for medium shot outings, but it is lighly packed. I mostly use the volume of the sling, to layer up and down and carry rain gear if it is in the forecast.
https://10colorstenkara.com/t/zimmerbuilt-strap-pack-belt-mod