Question about Enterprise licensing and Apple Business Manager
Apple did not make drastic changes to the Enterprise program at WWDC19. I don't know where you have that from. The only change I can think of is that they changed their terms to include that they can review internal apps - i.e. they want to be able to check your apps in case of suspicion of malware, bad practices, etc. This is a natural development when taking into account that there recently has been several public cases of companies misusing the enterprise program - breaking the rules - to install apps that are malicious or deceitful to customers.

You seem to be convinced that your situation is similar to the majority of other app developers out there - but that isn't the case. The majority of app developers, as far as I can tell, are playing by the rules.

It has always been totally clear that enterprise distribution is for distributing apps to your own employees. Not to customers. You should have been aware of this for years - I cannot see that coming as a drastic change that surprised you.

Apple makes it perfectly clear that this is not for 3rd party developers on the front page of the Apple Developer Enterprise Program web site:


  Your proprietary app must be developed by you for use on Apple platforms.


The fact that you have broken the rules for a long period of time, and it wasn't discovered by Apple, doesn't mean that Apple have allowed you to do so and suddenly changed their mind.

For your purposes you should really be distributing your apps through the App Store (for public apps), or through Apple Business Manager (for private apps) - or possibly through Ad Hoc distribution, if it's a limited amount of devices.