Looking for a small shoulder bag for wading and walking the bank  to hold a few 3500 boxes, some bags of plastics, pliers, leader etc. what’s out there?

IMO just a plain sling back or inexpensive backpack will do the job. Eventually the bait monkey will show up and you'll start using 3600 or 3700 boxes, and by then you'll know what you really want in a pack. 

For bank fishing I suggest a backpack. I have a Plano backpack, but the BPS stacked one looks great as well.

 

First off, slinging a tackle bag over your shoulder and carrying it will wear you out. But even more important, by using a backpack you can easily fish and move around at the same time.

 

But be careful - I had a tendency to overload mine and it got really, really heavy.

I purchased a Fishpond brand satchel a few years ago, and it is the nicest fishing bag I've ever owned.  It is a little different than the one below, but similar.  They are worth the coin...

 

 

https://uglybugflyshop.crazyrainbow.net/fishpond-blue-river-chest-lumbar-pack-peat-moss.html?id=42225010&gclid=Cj0KCQjwjryjBhD0ARIsAMLvnF_9sM44vdRnJZ6eT-3U3hFWvVALgHnYqUxRGdAaee_E7eAJ6NbF4uEaAsOeEALw_wcB

I use a Simms sling bag that is discontinued. Here is a comparable one from Orvis.

 

https://www.orvis.com/orvis-sling-pack/25FR-Family.html

I use a discontinued cabelas bag thats looks alot like their catch all bags but it has some extra features like a for forceps. They just cheapened it up and called it a catch all bag.  It will fit 4 or 5 3500s in the main compartment and 1 in each front and back pocket, but I dont know if the new bags have a rear pocket or not.  The 2 side pockets are good for terminal tackle.  I can carry a ton of tackle and for 12$ you cant beat it.  The sling bags are nice too, but they dont hold as much gear.  When my bag is stuffed, with a boga and microfiber towel on it weights like 7lbs.  Plano has some decent bags but the prices are up there and theyre a little smaller.

Cabela's Catch-All Gear Bag | Cabela's (cabelas.com)

Have you considered a vest . I walk  rivers wearing a vest and it works out out well . A vest , one rod and reel, pockets stuffed with small tackle boxes . Mobility catches bass.

I wear a fly vest and with the stuff I bring it's all I need.