I always found strange the fact that people make comparisons between format X and Y based on surface. :confused:
Nobody seems to take into account the aspect ratio of the formats involved. Think about it, 6x6 is 33% larger than 645, so it has to be better, right? No! The quality of the image remains the same, be it granularity, sharpness, whatever.
I think it's fair to say that 6x6 and 645 are 1,55 times wider than 135 and it's a significant advantage.
Just want to drop in the suggestion that the Fuji GS645s, which has a very sharp 60mm f/4, might meet your requirements. It handles like a 35mm RF and has a built-in meter. The only real limitation is the 1 meter minimum focus, RF cameras are not the best choice for macro work. You may be able to find one for around $400 US.
Pentax 6x7 can be hand held too, if (and only if) you use the wooden left-handgrip to stabilize them. Of course, they are huge, they are heavy on the shoulder, but they are good shooters with an ergonomy and prism view finder analog to 35mm SLR. And the 6x7 format... speechless ! I use mine with a 75mm for landscape and general photography, and a 150mm for hand held portraits. It works.
I can't agree with this! I've never had a problem hand-holding my Pentax 67 except when using the wooden handgrip. The handgrip sits out to the left of the camera body and gives a far less stable 'platform' than placing the left hand underneath the body/lens as one does with a 35mm SLR. I think a lot of rubbish is talked about the supposed 'difficulty' of hand-holding the Pentax 67: it is big and heavy, but the ergonomics are good and anyone who has handheld a large 35mm SLR (Nikon F5/D3 etc etc etc) won't find it a problem.