Z3's are the one Meraki product that my org has consistently had problems with. We thought they'd be a good solution for remote workers, but they just aren't reliable. I can definitely see how you are having issues if you have 20 devices on one, the recommended clients on the data sheet is only 5. We even have issues with one user on a laptop and IP phone. It's gonna be pricey, but you're gonna need MX64's at those retail sites.
@UCcert hopefully as I had already set 15.36 as the firmware before even powering it up we should be okay... We'd only ever intend on using it for 1 user with a phone and a couple of devices though I'd have hoped it would cope with more!