Where to get flight dynamics for a flight sim model?
Unfortunately I have no experience with how FSX models aircraft, but at a guess, it's model requires extensive experimental data from a real aircraft to truly get the right parameters.

And that's something no hobbyist is likely to be able to do. For that matter, it's pretty difficult for a pilot to do, since actually recording the relevant data is difficult, and some of what you need to know requires doing things with the aircraft you probably shouldn't do in most circumstances.

X-Plane's flight model and aircraft creation tool is far more forgiving. You still end up having to guess a lot of parameters, but they are generally less critical to basic handling.

All you really need to get a decent flight model out of X-Plane is a good set of reverence pictures, an eye for detail (so your model matches the geometry properly), and ideally the correct airfoil profiles and engine specifications.
(Primarily the thrust and power)
For the most part, good reference models and diagrams, and the information you'd find in the Pilot's operating handbook is enough to create a decent flight model in X-Plane.
It certainly won't be perfect, and you'll probably have to tune it, but it's a much easier task than getting that data needed for an FSX model.

I have the good fortune of being a student pilot, and as such I decided to attempt a model in X-Plane, and found that while it was far from perfect, (and needs a lot of improvement to be a 'good') model, it's behaviour was much closer to the real aircraft that I fly regularly than I was ever expecting given how much I had to guess.

I had to guess everything from the aileron deflection angles to the propeller geometry, wing airfoil choices and more, and still the resulting model was only slightly off from the real thing insofar as I know how the real aircraft flies.

I guess that's not an overly helpful answer in a direct sense for an FSX flight model, but I fear it just isn't going to be at all easy to find the information you need to make a flight model that isn't fundamentally broken in FSX, let alone accurate.

X-Plane is just far simpler to work with when you don't have a lot of information...

Whether that's worth the downsides of X-Plane (especially the consequences of switching if you already have a heavy investment in FSX), I don't know.
But it's worth keeping in mind if you are particularly fond of amateur aircraft design.

(It's even plausible to create fictional designs in x-plane for which no real-world data could ever exist in theory, and still get a good idea of how such a design likely would fly if it did exist.)

As for FSX potentially having better flight models in some cases? Maybe. But this is likely going to be the flight models of expensive add-on aircraft models that were made with the help of the manufacturer and pilots qualified to fly the real aircraft.

That's not going to help you any if you don't have access to those kinds of resources.

A hand-tuned model matched to exact real-world data may well work better than a physics based model if you have good source data.
But if your source data is lousy (as it is for most of us unfortunately), then the physics based model will be much more reliable most of the time...