RE: Best way to integrate with AX.
You seem to be focusing on direct web service calls. Do you have any reason for it? They're definitely not the only way of integration and they have quite a few limitations.
Please make sure that you collect and analyze requirements and choose a solution suitable for these requirements. If you choose a solution irrespective of what you actually need, you may cause a huge amount of problems to you and your clients.
For example, if you need to exchange a lot of data with web services, performance would suffer. Using data management APIs may be a better choice.
Or let's say you'll generate a lot of requests and overwhelm the ERP system. You don't want that! A solution may be using asynchronous processing (batch, message queue), or throttling.
Similarly, what if you want to call a web service when D365FO is down for deployment? You'd get an error. Will your accounting manage this situation and resend the message later? Maybe you would rather use a message queue.
Or what if you want to get notified about something happening inside F&O?
Maybe you don't want to implement such things in either the accounting software or the ERP and you'd rather use in integration platform (such as BizTalk).
Maybe you'll find out that no single technologie is perfect for what you need and you'll combine them. For example, the package API for normal data transfer, OData actions or custom services for a few calls that must be in real time) and message queue populated with business events for notifications from F&O to your application.
Again, the key piece is to understand business requirements. Forget about implementation details until you understand what you need to achieve.