The NX registry designation was used for experimental starship designs, basically a starship class prototype, not for ships that were used to test experimental technologies.
In other words, the NX meant that the whole new ship model was under test, not the single technologies that it carried on board; and the premise that the Discovery is an experimental ship, instead of a ship used for testing experimental technologies, is wrong and not supported by any evidence.
There is no indication that the USS Discovery was the first ship of its class and therefore considered to be a prototype (so being registered with a NX designation), as a matter of fact we have some clues that imply the contrary:
- we know that the USS Discovery registry number is NCC-1031, while its sister ship, the USS Glenn, had NCC-1030; this implies that the Glenn was launched before the Discovery; this eliminates every chance that that Discovery was the first of its class;
- we also know that both these vessels were Crossfield-class starships; there are many examples where the class name was also the name of the first ship of that class (Excelsior, Galaxy, Prometheus, Defiant); so we can safely assume that the first ship and prototype belonging to the Crossfield-class was the hypothetical USS Crossfield, that would have been the ship to be registered with a NX designation.