Why did you choose LF to begin with? If it had to do with tilt and shift options, then you won't be happy with MF gear unless you change your expectations. Do you print at very large sizes? Then you may have to change your expectations. There are some very sharp lenses for MF, and some fine films like Acros and TMax100, that will allow fair enlargement without resolution or grain being much of an issue. But only you know what your standard is.

Unless you specifically want to pursue square (which in itself is noble), I do not think the 6x6 is the most practical use of film real estate. Closest to 4x5 in aspect ratio is 6x7 followed by 645. If you routinely print on 8x10, 9.5x12 or 11x14 paper sizes, then 6x7 is the ideal format that requires virtually no cropping. The more efficient system for me is 6x4.5, which prints straight to 12x16 and with minor cropping to 8x10 and 11x14.

In 6x7 you have the Mamiya 7 rangefinder with its expensive and superb lenses, the Mamiya RB and RZ, and Pentax. In 6x6 of course Hasselblad, Bronica and one or two others. In 645 there's Pentax and Mamiya, and Contax as a higher end system.

The Pentax 6x7, which is more SLR-like in handling, is an option with superb lenses and decent used prices. I would recommend it for field work above the RB/RZ type cameras, and above the Hasselblads too, perhaps. The Pentax lenses are more geared toward outdoor work in terms of their rendering, the available focal lengths and ergonomics, whereas I think the Hasselblads and Mamiyas excel at people photography and in studio. It is a really good idea to try out a system hands-on before committing to it, as each has its own quirks and output quality that require getting used to. I find the barrel focusing for instance much easier for handholding than the bellows/rail type. Others may see it differently.

Note: I do not own any Hasselblad equipment, so my experience there is limited, and my opinion is based on what I've seen others produce with it. I do own Mamiya RZ Pro II and Pentax 67 II systems, though. If I had to keep one camera only, it would be the Pentax. In 645 I have a Mamiya 645 AFD II and since recently a Pro TL too. The lenses that I own are all good to excellent. I have yet to try a MF lens that is a real dud, although I do believe there are a few here and there.