What kind of effective speed did you get with Diafine? Diafine seems to work best, in terms of speed anyway, with traditional films. Tri-X is effectively faster in it than TMZ is, so I never tried it with D3200.
I don't find TMZ "particularly grainy" with T-Max developer. It's grainy, sure, but not that bad. Very nice at 5x7 and not really obtrusive at 8x10. I'd hesitate to print larger from the combo though, unless the grain helped the subject.