Made a decision: Ordered a Pentax 645N for squishy reasons: 1) AF (which Mamiya has, too), but also 2) already had a manual Pentax 645 A 150mm on hand ....which I wasn't thrilled with on a Fuji GFX, so it kind of moved me there to either unload or consider it here for an interim. Also picking up an AF 45mm f/2.8 too as a cheaper AF than the other options, AND if the 645N proves itself, might switch the A 150 for an AF 150mm and possibly an AF 75mm. Not necessarily in that order, and certainly no rush. But 3) Liked the EXIF film tagging that I think only the Nikon F5 also has - but could be wrong. Finally 4) Didn't want swing for the 645N2 at this point as that only adds MLU but runs another $200 to $300, and if that's really a factor I should be shooting 4X5 or mirrorless or trade for the N2 later.
Why not the Mamiya: Getting the Mamiya parts together reminded me of the Rolleiflex 6008AF series which was kind of a bitter experience for me.... and just too many aging parts.... all of which are old and fiddly. Yes, they were wonderful when they worked... that just wasn't often enough. Rollie's tight tolerances were beautiful and the lenses, too, but there's no repair... and sometimes things go swish-bang-jam as in "that's didn't sound good." Heebie Jeebies, and yes, that drops out a lot of possibilities 'cause assembling all the gear you need - even basics like a cable release - just adds time and $'s that a little simplicity cures. Mamiya 6 appealing as it is, seems just more spendy than necessary and suffers the RF issue when you go to portraiture where the smaller focus and framing square makes you wonder "is this really a good idea?" - although I'm still reserving that possibility as a "stop gap option" if the 645N fails to grip. As to Hassy... don't even get me started. I've dropped big $'s on lenses before chasing the Zeiss "glow"... and they're beautiful, but I didn't see getting lost there. Increasingly keeping my "kits" light... 3 lens max (though the carry may be less) and devoting my energies to what's in front of the lens, on the lens, inside the camera and mostly in my brain and trying to get that into an image. FWIW, GFX is SOOOO good that I'm over the Hassy.... I may also be over the GFX as it could be overkill, but that's another story.
Finally yes, I like square ...but only to a point. It's a compositional challenge unsuited to architecture and landscapes IMHO. YMMV. But I'm really looking for a 120 size camera that can be a bit faster than 4X5 with a roll back. Doesn't have to be a race car, but AF is not just nice to have any more as my eyesight geezes. Beating 4x5 in set-up or carry time ain't hard either. Photography doesn't have to "Everything Ever-owned (Carried) All at Once". Rather leave behind too much (even in the car) and missed it than carried too much. Yes, I'm a geezer and thankfully regret is something you've gotten used to by now... so it bothers you less.