Big Enterprise here. I recommend reviewing the App Distribution session from last year noted by Vanson. For the scenario you describe in the question, I recommend B2B Custom App distribution. I do not recommend using your customer's Enterprise account for many reasons.

You need a standard dev account that can target the App Store to publish a custom app. Enterprise accounts only target enterprise signed apps.

VPP supported this feature but this was lost in move to ABM. We have tickets filed for this as well. Note ABM does not support international Locations at present so I recommend also highlighted that in your request if it’s relevant to you. Managed Apple IDs and Enterprise iCloud hosting are another feature set that would benefit from international Location or Multiple Organizations and location support in one ABM instance.

File a feature request Derk. We are in similar situation so that is what we are going. The more requests the better.

As the stack exchange post notes, you have to complete Banking and Tax details for your dev account before the Custom App distribution option becomes available. It’s odd since you likely will not be charging for your app but I suspect it is still required for Tax tracking purposes.

in iOS 13.4, Shared iPad support was added for Enterprise but the feature can’t be turned on unless the device is supervised (or Auto Device Enrolled) and your MDM supports Shared iPad enablement for Enterprise.

Good question.