I’ve got a 26" x 2" Big Apple on a Mavic X717 on the back of my Bullitt. It seems to be really sensitive to tyre pressure when it comes to lateral stability. There’s this sweet spot of a good supple pressure that won’t squirm in a corner, but as it slowly leaks air as tyres do, it becomes more and more squirmy.
I suspect this is the point of Big Apples; fast rolling supple tyres with a supple side wall, and maybe I just need to keep the pressure up. The alternative I can think of is a wider rim. The clearance is already a bit tight, but it could work. A quick check found that Stans ZTR Crests are pretty much the same ERD as the Mavic, so maybe I could try tubeless, but on a cargo bike? With a non-tubeless tyre? Or just pump it up? Deal with the squirm? Some other rim?
I’ve got Stan’s Crest with a Compass Tyre running tubeless. It’s a slightly narrower setup to yours at 650b X 48. My pressure is somewhere in the 40 psi range … Give or take as my pump gauge isn’t accurate. I do get the squirm in the corners but I’ve gotten used to it after realising that it’s how the tyre behaves and it doesn’t affect the steering or my stability. My Stan’s were a complete wheel set and they were rated to 90kg rider weight. Perhaps double check the rim weight limit to see if it will suit the cargo bike.
On my commuter I run Big Apples (700 x 60c, liteskin) on 28 wide Sun Ringles. The sweet spot for pressure IME is around 45-50 psi. Below 40 they get squirmy, much above 50 they ride like bricks. Mrs C has 26 x 2" on her Van Moof, with narrower rims. 45-50 is about where it’s at for her bike too (& she weighs a lot less than me)
Mmm that’s a real narrow rim for the tyre. Probably your best option is to replace it, cheaper option would be to just run a narrower tyre, perhaps a 1.75, meh option is to live with it and maintain your tyre pressure.
If replacing, the ‘mid fat’ rim from Velocity looks like a good option - 39mm internal width.