On the low light, that is true. It's just a point a made, but maybe a useless point. Sorry for that.
Maybe me learning to use a flash could do wonders

About the Pentax 645 is not my taste, it's very bulky unpractical looking. I'd take a Pentax 67 any time over the 645.
I think the pentax 67 is for a future purchase.
Right now it's between the Fuji Ga645 (60mm f4) and the Fuji Gs645s. The Bronica Rf645 is nice too! But seems to be more expensive like the Mamiya 6.