Look at the difference in acreage.
645 is 2.7 times the area of a 35mm frame.
Bronica glass is likely equal to any glass that Nikon poured.
6X9 is 4.5 times the area of a 35mm frame.
What frame size are you considering? You said SLR MF which, for the first level of MF, would be the ETR for 645, of which the most evolved is the ETRSi, which includes everything a Nikon would do, including Mirror Lock Up on that last model. The AEIII prism is the most evolved meter for the ETRSi.
There is an obscure wide panoramic film back for the ETR bodies that shoots a 24X56 pano. Also, there is a standard 135 35mm back for the ETR bodies that shoot a 24X36 frame.
The square 6X6 SQ-Ai is the most evolved square format and both of the 35mm backs are available for the SQ-Ai body.
The GS-1 is 6X7. Don't know about the 35mm backs.
Each body has it's own series of lenses with good selection through the focal lengths, and a couple of zooms...
Do you use flash, because ALL Bronica lenses are LEAF shutter in the lenses, allowing flash sync at all speeds.. every lens.
Lots of used inventory in Bronica, and the prices are VERY reasonable.
A good ETRSi with the regular 645 backs and the 35mm back would obsolete the need for the Nikon. The ETR system is modular, but I personally don't consider it all that much larger than a pro SLR film camera.