Hello Eric,
What you can try to test is
* Splitting up your total invoice amount into 12 equal installments each consisting of a total amount of 142.5
* Use the standard interest calculation functionality available in AR. As this functionality does not know interest rate free periods you have to test whether and how you can work with the effective and expiration dates and multiple rates. I have not tested this based on your data but could imagine that it will work even though the setup might be complex.
* Run the periodic interest calculation in AR to get the interest calculated and post it to the customer account.
I guess this is pretty much what you can achieve in Standard AX. If the whole process becomes to complex / difficult to handle, you might need to implement a system modification.
All the best,
Ludwig