IMHO you would be a lot better served by using Arduino boards. It is a very mature ecosystem and there is a LOT of documentation and sample code out there. This might involve a bit of up front effort in learning the basics but will pay off big time in the long run because you will have a system that you can change, add to and re-program as often as you need to.
There are several Arduino variants that can act as a fully compliant USB HID joystick with just a few lines of very simple Arduino code.
Have a look here to get some ideas.