I'm not familiar with "flash" as a component of a stucco wall. You gotta help me with the term.
The most important aspect of patching stucco is getting the old stucco removed back far enough to expose undamaged layers of the moisture barrier(s) (flash?) sufficient to layer over and under your new layers as appropriate, and flashing your new barrier layers correctly at the window flanges. If all that's done correctly and effectively (no mean feat), the problems with cracking stucco are of little consequence in the overall picture.
No, I'm not aware of any stucco product that doesn't crack. Another part of the secret of a three-coat stucco application is to make the scratch coat richer in Portland than the brown coat. The scratch coat will be harder and more likely to crack; the brown coat will be softer, still likely to have some cracking but the cracks won't coincide with those of the scratch coat. The finish coat can be whatever you have on the rest of the house in the case of patches and the color won't matter on accounta you'll hafta paint it all to make it match anyway.
I like to paint a slurry of thinset on the raw old edges when I stucco the patches. I can't prove it helps it to bond much, but it's old habit. Chances are you're gonna get cracks there anyway.
I generally use pre-bagged stucco mixes for patch work. Quikrete makes one (which I've not used), as do several others. LaHabra is a brand that is readily available in my part of the country. It's very good, has all the right stuff already in it, including the fiberglass reinforcement, and all you add is sand and water to taste.
As with tile work, paint, and many other parts of the building trades, in stucco, preparation is everything
. The stucco itself is nothing more than protection for the underlayment and a way to make the wall more attractive.
My opinion; worth price charged.