The 6x7 format is a full 400% bigger than 35mm and a good chunk bigger than 6x4.5 and is, as it is termed, the "ideal format" as it requires no cropping to fill a page. And anyway, you could crop it any which way you can to imitate another format. The 4x5 format, while similar in its aspect, is superior if you are technically adept (it can be demanding, especially in low light) and know how to wring the very best out of its imaging quality — getting into it just because it looks nice, without any prior exposure to it, is silly. It is also believed to be the first format to suffer as film dwindles. We're now seeing that with Fuji's discontinuation of several lines in 4x5, along with what remains of Kodak. 4x5 is also much more expensive if you do not process C41 or E6 yourself. Not a problem for B&W in your own darkroom. All things else considered though, it is the format to provide you with very large, crisp and startlingly beautiful prints (I'm quite happy and thrilled producing 28x36cm (11inch) prints from 6x7. On an economy of scale and availability of wall space for hanging prints, that's as big as I go. Each print is $280 in cost from scan, colourimetrics, inversion, proofing to final print and lastly frame-up. Anything bigger is not in my financial league.
What's so really special about the 645 beside automation? Have you the skill to do additive/subtractive/basal spot / incident metering to be able to step in, say, in low light or challenging mixed light, when the Pentax 67 TTL meter won't do? You'd certainly need those skills using 4x5 (plus a working knowledge of the Zone System for metering). Or rely solely on gimmicky electronics and multi-pattern metering and whiz-bang film advance. Why is that necessary?
With B&W film, you do not need all that electronics: remember your interest in 4x5? Where are the electronics in a Linhof, Wista, Toyo...whatever else? None! Nada! Creative spark is in your hands guided by your grey matter, a meter and a keen sense of observation.
How irksome: where is the proof that the 645 is potentially more reliably built? Over what? What do you think of the Pentax 6x7 / 67 bodies are more than four decades after these oddly-termed "badass" boxes came out? Not exactly clapped out, probably worn, and justifiably so. More electronics does not necessarily equate to better reliability (or for that matter, better photographs!). 67s can be damaged, most likely through the user incorrectly mounting/dismounting prisms and lenses (the coupling chain can break if a procedure is not followed), wind-on lever can be stripped from aggressive winding, locking wingnuts on base where film spools lock into can be broken...
Of lenses, I don't consider the 105mm 2.4 to be a stellar performer. On the other hand, a few pentax SMC lenses are eye openers, even though the best examples still command a fairly steep price. I nominate the 45mm and 55mm f4 and slightly behind them, the 165mm LS. I did my homework very carefully selecting these. All my stuff came off an estate on eBay, mint. No plans to buy others: consider the weight of a 67 kit with three lenses! It
is a chore to cart around that kit with a tripod. Especially uphill...
Mirror slap... What slap? Being deaf like me has advantages. Aside from that MLU gets around most mirror/shutter-induced whack. The rest is down to taking care.
If a full-blown 6x7 kit is weighing (heavily) on your mind, there's always
this.

So what format will it be?