Does your customer expect to have more than the dozen or so iOS devices, or is that it? Because if it's only ever going to be a dozen phones, truthfully Jamf Pro is probably overkill. I would never normally steer anyone away from the product, but it's a robust tool with lots of features that you just may not need, but you will end up paying a fair amount for those unused features.
Jamf also makes Jamf Now, which is much more affordable and simpler. It's geared toward SOHO environments that have a modest number of mobile devices (or Macs) that they want to manage with settings and push some apps from the App Store, etc. I would at least take a look at that. You can set up a free Jamf Now account and they give you a couple of seats for free, so you can test it out at no cost.
Regarding Apple Business Manager, that is never a requirement for any MDM product that I know of. But it's a good idea to have it if possible as it really helps automate the enrollment and setup of those devices when they are new or get wiped and re-setup. Even if your vendor doesn't participate in ABM, you can look at getting a Mac and the Apple Configurator 2 utility to manually enroll them into the program. https://support.apple.com/guide/apple-business-manager/add-devices-from-apple-configurator-axm200a54...
