Well, are you forgetting the Calty center in California that Toyota has for engineering (Camry, most of the Lexus line)? Then there’s Honda R&D Americas, where the Accord is developed and is the ONLY place in the world that the US/JDM Accord is developed. The same facility developed the FCX Clarity and the Element. Nissan had an R&D center up in Michigan for decades; Hyundai did as well - not sure if those have moved with their plants/operations.
Almost all of the transplants have test tracks or use test tracks out in the desert southwest of the USA; many rent US tracks for development and testing, many do real world testing here on US roads.
And you’re forgetting Honda Racing Corp - which is Honda’s racing division, mostly run and staffed by Americans (which is why the HRC colors have been Red White and Blue since the 70s.)
In actuality, the Asian transplants have a hell of a lot of R&D, testing, and engineering facilities here in the US. The Europeans less so (the last ML wasn’t actually designed here in the US, the new one was partially developed here.) Heck, Honda even is developing airplanes in the US - wholly in the US.