I know you were a trained and skilled tailor before you were a mechanical engineer. On the metaphor, we don't know whether almost all of the time the ride will in fact be all downhill, and the task is mostly steering the operator can manage. But, in the note quoted below, there's an explanation of why some old iron is capable of a wider range of performances, at least on a light duty cycle, than many specialized industrial machines. That and OP's post a while ago about impingements on his businesses growth lead me to think he's not in the market for a Consew 199R, truly the industrial, heavy duty-cycle -- version of the multi-purpose old heavy home iron ZZ.
On your affection for and the the undoubted greater capability of any 3/4 horsepower motor --clutch or servo -- I just looked up this tagline I first saw 15 years ago: (and am surprised by who it is attributed to)
Aerodynamics are for people who can't build engines.
Smilies, of course, and with the greatest respect.
OP: Here's a communication I got from a serious and more qualified amateur than I am, who like me favors not just old iron, but for zig zag, old iron that is "end loader", with the bobbin on the left and the entire shuttle mechanism and its enclosure moving for zig-zag. (Yes, that's a lot of mass to oscillate, compared to just needle-bar, feed dogs, and shuttle.) Notice the thread and needle size, and a suggestion of a good Singer to look for locally, one which would be capable for several layers of 1 oz nylon, but also the occasional strap and harness work.
My primary focus is on using domestic machines for "near industrial" purposes. The old S[traight]S[titch] Singers (and probably others) would routinely take up to size 21 needles and v92 (heavy) thread as the old machines had to sew the occasional horse collar as well as clothing. With the advent of ZZ and the rotary hook, everything changed. Since in ZZ with a rotary or oscillating hook, the hook has to grab the stitch early on one side of the ZZ and late on the other, the tolerances are more critical than in SS where the hook always grabs the stitch at the optimum point in the middle. Therefore in modern ZZ machines, the play needs to be a minimum and the needle size and thread size need to be in a tighter range than in the older SS machines.
That is how I got into this.
The solution, if it can be called one since it is a compromise, is to use a transverse hook which moves and is always under the needle, even in ZZ. Therefore the tolerances can be opened up just like the older SS machines.
The Sailrite semi-industrial sailmaker machine is a good example. But Singer did make one ZZ machine like that and the last good Singer for this purpose is the 237. It has all metal gears and a translating hook and takes up to size 21 needle and large thread just like the older Singer ss machines. The Singer 237, and I have 3 of them, were made in the Monza Italy plant and are the last of the "good" Singers for this purpose. Since then they are all clothing machines with a max needle size of 18 at best and generally plastic gears.
The point here is that the Singer 237 machines can have more end play tolerance etc. but the newer machines combine the end play, needle size, and thread size to determine what can be sewn. So reduce the end play as much as you can if you want the other variables like needle and thread size as large as possible.