What type of NAT algorithm are you using for your NAT here? E.g. full cone, symmetric, etc. Such details can be useful for testing. Currently I'm having to code a small script to simulate different NAT behaviours so I can test trying to traverse them easily. I looked for a way to do this kind of thing with cloud networking ages back but they were no where near as configurable as needed. Normally the details were hidden. IDK what they would be using in that case.
The other thing is the port allocation algorithm. Many home routers try to preserve the source port (assuming its available.) But there's other algorithms that could be used here like independent delta +1 (increment each new mapping by +1) and others. IDK if there's a target market for something this specific or niche. But thought I'd put it out there.