I agree the times for D3200 (and Kodak's times for TMZ too) are too short. I develop both for the times given for one stop more speed, generally shooting at 3200 and developing as listed for 6400. I do it all at 24C too but don't get anything like your grain!

"Over develop D3200 for grain" is certainly a viable way to get grain, but you will get a lot of contrast too which you may or may not want.