Good Morning, Christian,
There are a lot of variables here, and I suspect you'll receive a number of suggestions.
My first thought is that for the size of prints you mention, you're stretching the limits of 35mm, especially with 400-speed film. I'm not referring only to grain, but to overall tonality. With many medium and fast films, 35mm can produce very good 8 x 10 (18 x 24 cm?) prints and fairly good 11 x 14 (24 x 34??) prints. When the stars align properly (slow film, the right developer, excellent lens, good darkroom technique), prints of 11 x 14 or slightly larger can still have good tonality and look great, but it takes a lot of attention to detail.
Developing more is unlikely to improve things; the increased contrast will probably make printing more difficult with most subjects.
Easiest first step: Shoot with the Mamiya. The film size (6 x 4.5) isn't nearly as helpful as a 6 x 7 negative would be, but you should see noticeable improvement over 35mm.
Better first step: use the LF monorail whenever possible.
The split-grade printing may help with some negatives, but using a bigger film size would still be my basic suggestion.
Konical