OK! There's the difference. Most copies of the 24-70 f/2.8 MkI were unmitigated pieces of
shirtwhen compared to the prime beating MkII. I'm not the only photographer around CR (and elsewhere) who ploughed through five or six 24-70 f/2.8MkI lenses over a number of years in search of one of the rare good copies.
As a signed-up, paid-up life member of the Z Team, I'd also choose your primes, the 24 35 85 combo over the old MkI zoom for an important job. My first day shooting with the MkII zoom was one of the happiest days of my life. To say I was gobsmacked by the quality across a broad variety of situations is almost an understatement.
The problem with the 24-70/2.8 Mk.I is not that it was a bad lens. It was a very good lens when it was properly adjusted. They were generally fine when fresh from the factory, but would go out of adjustment with regular use. Within a year or two, even the good copies could become bad copies. The more one used it, the more likely it was to go out of adjustment. A heavy user of that lens was well advised to send it in for annual adjustments, even before the blurries started to show up. On the other hand, a light user might not ever see it go out of adjustment. Related to that, the adjustments themselves were not that easy to make. And finally, even when well adjusted, some users reported that the lens liked to focus on a distant background a little too often, no matter how carefully it was focused on the subject. It was a generally useful lens but with some reliability and maintenance issues.
The Mk.II version is better in all respects and truly prime-like in quality. That said, any lens with a complex zoom mechanism is likely to eventually need a tuneup with heavy use. Primes, being simpler mechanically, generally don't need as much attention.