I think I have come to the conclusion that Microsoft has made it impossible to use any REST API calls without AD FS or an on-cloud instance of Dynamics 365, but I figured better to ask first. I find it frustrating that Microsoft continues to ensure their tools are the most difficult to develop against :-)
Assuming I have to use an on-cloud instance or on-premise with AD FS, is there a way I can gain access to a cloud instance of Dynamics 365 for development purposes?
My company is broadly using Microsoft 365 for our Office / email solutions, but internally we abandoned Dynamics CRM for these extreme complications, opting for easier to work with solutions. That said, I now have a customer opportunity to build an integration against Dynamics 365 ERP, and I am recalling all the reasons why I was happy to drop that solution for our own purposes :-)
Sorry for my ignorance,
Rod