I share your enthusiasm for the film but I have to object to your characterization of Fred "attempting" to sing and Judy "attempting" to dance. Fred introduced many classic songs in his movies and got high marks as a singer from the writers of those songs, to the point where there was competition to write for him. There have been several collections on vinyl and CD over the years; maybe one of them would change your mind. As for Judy, it was common for choreographers and dance partners to comment on her natural ability as a dancer. Some insisted that she could learn a number after only one runthrough and she more than held her own as a dancer with both Astaire and Gene Kelly, the two major male partners of her era. I don't agree that it was a case of "feet vs. voice" in either case. They both had it covered.
I totally agree about Ann Miller and that it was evident that she loved what she did.
I think Fred Astaire's best screen partner was not Ginger Rogers but actually Bing Crosby. Suggesting that Fred was a singer of that quality is a stretch...and likewise suggesting Bing was as good a dancer as Fred is also a stretch. But when you put them together, you get the whole deal. It would be like putting Ginger Rogers in a movie with Rosemary Clooney. One can dance well and one can sing well. But neither one is totally skilled at both.