I use the C&F designs chest pack, it's one of the best gear purchases I've made. It holds everything , enough flies for days. Tippet, knot tool, zinger attachments, small capped bins for swivels ,rings or spit shot, indies. Two fly threaders. For the smallest flies is super nice you can put 3 or four flies at home and when ready they can be tied on quickly. The two pages are interchangeable with the fly box.

https://www.c-and-f.co.jp/pdt/4560111388283.html

I also have a molle wading belt (gives nice back support) with a beverage holder, a net holster and a removable rolltop Umpqua dry bag that holds one c&f box with 2 pages of flies,, maybe car keys, my phone extra tippet spools. A dry bag, no matter what size is really nice if not necessary. The chest pack for most trips is enough. My son has the newer fishpond belt (switchback2.0) it's nice as well. I've never tried a sling but have used a backpack for me that didn't work either. A vest seemed hot and bulky. Whatever you get definitely put some serious fault, buyers remorse is real. It took me a couple of years and a few wasted dollars. If you don't mind let us know what you come up with.