I have a fairly new mainsail so I know the issue of unruliness well. Whereas I can’t speak to the ease of the system you are looking into, I can, perhaps, add something to the discussion here of alternatives. I’ve crewed on several boats with the stackpack system and considered one for me. Yes, it solves one problem or “headache” of securing the unruly mainsail, but it creates others. They’re just not in my experience the smooth, easy, one-step
alternative they look to be. For one thing, they still require use of lazy jacks so what actually is gained? I note above that Tally Ho finds his system very convienient.
When I installed the new mainsail I up-graded my lazy jack system to four pairs of legs instead of three. They are fully adjustable so I can slide the guides for the legs on the boom to where they need to be. I can fully loosen the legs when hoisting, then tighten them after hoisting to produce a very “tight enclosure” so when the mainsail is dropped it pretty much stays atop the boom, even if it might not “flake” itself coming down the way I would want. Frankly, I don’t recall seeing a mainsail properly flaking itself into a stackpack either. But you don’t have to “deal with it” as much b/c you can just zip up the cover and the sail is put away in whatever the condition it came down, etc. So looks great. And that is convenient.
But since your principal issue above was “dousing” while essentially single-handing, there is effectively no difference here between a four-leg adjustable lazy jack system and a stackpack system. If the sail comes fully down as you wish, it will lie on the boom either way. It’s true that for securing the sail you would need to strap on the sail ties lacking the bag to keep it secure after coming down.
If I may continue, the concern about “proper” flaking is another issue. I’d welcome insight there from the stackpack users. To do this on the Bavaria when the mainsail does not have to come down fast all at once, I flake it down single-handedly using a halyard parking bracket at the mast. I can stop off the halyard at the mast where I wish (say one-third down) and then tend to flaking of that much sail, proceeding fwd from the leech. B/c we have a dodger it helps to swing (and secure) the boom outboard some to where I can stand on the cockpit coaming to reach the end of the boom (cf. Post #9), and to apply the sail ties. Once secure, return to the mast to lower more sail, stop-off, flake, tie, and etc. again.
No, not perfect. I still have to lug the mainsail cover back on, which is a chore.

So for ease of operation, a zip-up probably has it over the old fashion way but does not add much regarding “dousing” while single-handing.