You're problem is really very simple. You're using a silly toy tripod head. It would be dishonest to phrase that more politely. It has nothing to do with the P67 shutter itself. Yes, employing the mirror-lockup is often helpful (always if under 1/60th in my case). But you need more actual mass under the camera. And avoid ball heads like the plague. Get a decent pan-tilt head like the larger low-profile Gitzo instead.
Faster than around 1/60th, the mirror doesn't slap until the shutter has already done it's thing, so there is no mirror impact to the shot itself. My older brother actually sold Rollei gear and had a couple of SL66's himself, with their extremely gentle mirrors. He demonstrate them by setting a dime on edge atop the camera, itself atop a table, and then trip the shutter. The dime wouldn't even fall. With a P67, it would land half a mile away, and a seismograph in the next county over would probably record 3.5 on the Richter scale. But he preferred borrowing my p67 for its superior ergonomics, and even got sharper shots with it routinely.
Heck, I even often shoot the big heavy 300EDIF tele lens on my P67, bolted DIRECTLY to the top of my smaller Ries wooden tripod via the rotating lens collar (no intermediate tripod head, and none needed), and routinely get extraordinarily crisp shots. Anyone who doesn't, simply has the wrong technique or wrong support system. And if that offends someone, so be it. It's the truth. Every time I hear a gripe about poor performance with the 300's, it turns out they're using some toy tripod setup and don't even have clue how to properly support things, free of vibration. (The older Takumar 300's aren't as well corrected, and are harder to support steady, due to lack of a lens collar mount.)
For teles shorter than that, or other P67 lenses, I can even use my lighter CF tripod with complete confidence. But I prefer the height and stability of the Ries whenever possible, or else my bigger CF one, just like I use for 8x10 format.
All the later P67 lenses are top notch in terms of sharpness - some of the early Takumars too, like the 105 /2.4. Don't blame the lenses if something is not sharp. Their later teles of the EDIF class are probably the best MF teles ever. Astrophotographers prized them. P67's were also prized by aerial photographers. And unlike the RB system, you have several fast lenses to choose from. There's nothing second rate about the P67 system whatsoever. My first P67 body bought 45 yrs ago still functions perfectly; and it's seen some hard use.
Yeah, I prefer my Fuji "Texas Leica" 6X9 RF's for handheld shooting, or compactness; but those are fixed lens cameras, and not a full system like Pentax has. And the ubiquity and abundance of lenses and spare bodies and prisms out there is one big advantage over the Bronica system, which is rather uncommon. M7's are nice, but there's no "normal" focal length lens, and only a token tele presence; and they're getting damn expensive even on the used market.
I have nothing against RB's. But its getting harder to find elephants and mahouts to help carry them. Heck, my 8x10 kit is probably lighter.
Metering issues? I always use a handheld Pentax spot meter anyway, for all my cameras. I haven't actually used the TTL meter prism for my P67 in decades, though it still seems to work fine.