Yes I loved the RDX 500 mp for a while. In terms of Yonex, from what I have hit with, I would say the RDiS 200 and EZ Xi 98, although both are stiffer and from all reports I would say the VCORE 97 310g.
Outside of those the IG Rad Pro has that sort of flex, but is string sensitive, so you might not get that buttery feel with a poly in it. The only other that I have stuck with over the years with that pillow type feel is the PST. The Redondo mp is also very close to the RDX500 HD version IMO.
Ai 98, Vcore Xi 98, PS 97, PS 97LS (shoot out)