I have had a softside tacklebox for a long time. It holds 5 large trays and has sidepockets but its large and not handy to carry shore fishing.
I want a backpack tacklebox large enough for a tray with crankbaits, one for worms and softbaits and s tray with hooks and sinkers. I mostly fish for largemouth bass but also crappie and bluegills.
Anyone have experience with them and can give me tips on what works-or what doesn’t?

I guess mostly what i need to know it what size and which trays work best.
I have seen some advertised with smaller trays that lat flat and some with larger trays that are dropped in vertically.
I want to carry gear to some small creeks and ponds that I can’t drive to.

I have one, bought off of Amazon. I can swap the trays when switching from fresh to saltwater. It allows me to easily carry my crap way out on jetties, or quickly load everything on my kayak.
I don't have it close by, but f I bought again I would get the one with the Velcro strap rod carriers.

What brand and model, or does it matter?
I have seen online prices from $40 to over $100.00

Do I need a “fishing backpack tacklebox”, or just s small-medium backpack to hold plastic lure trays?

I have one of these and it has held up so far.
I carry 2 9"x5" boxes in the main compartment, but I bet 6x12 would fit as well, the outer smaller pocket holds a plano mini magnum and another 4x6 inch box easily. I have a light rain jacket in the rear pocket, and the water bottle pocket on the bottom holds a gatoraide bottle.
I use it for wade fishing, and it can be swung around front without taking it off to access the pockets.
It is chinese, so I cannot vouch for the quality of each one.

Go and get you any run of the mill backpack that you're comfortable with. I still just throw on an old school backpack because, why not? As long as it holds your gear you should be good. Also, bigger is not necessarily better. The larger the bag, the more you'll fill it, the heavier it becomes.

I have the BPS backpack that I carried for co-angler use and I still use some for bank fishing. It is ok and has held up, but I'm looking at other options because I don't like only being able to fit four 3600 size trays in it. I always ended up with a 3700 size that I use to store crankbaits sticking out of the top compartment. For a while, I just loaded up my Maxpedition Typhoon with two 3700 trays and plastics and carried that bank fishing, but it was really stuffed in there and kind of a pain to get stuff out. Any pack will do as long as it meets whatever size and organizational requirements you may want. For something purpose built, I would check out the links below. I picked those two and am considering trying one of them because I prefer the 3700 size trays.

https://www.tacklewarehouse.com/Flambeau_Heritage_5007_Backpack/descpage-FHBP.html
https://www.tacklewarehouse.com/Lews_Mach_HatchPack_Backpack/descpage-LMHPBP.html