Only just noticed this.
Got 8 industrial sewers (at last count). All walking foot. Needle feed machines (bobbin accessable from top) don't require 1 to turn their wrist inside out to get to it. Got both kinds. Advantage of course with bobbin underneath is if you are working in the middle of something big, can load a fresh bobbin up and keep sewing. No need to remove work.
Never used a zig zag machine, just plain lockstitch sewer, so cant offer anything there.
Once one has a long arm, wonder how you did without it. Picked up mine (twin needle) on the secondhand market. But looking at the machines you are playing with, prob a bit on the heavy side. Head alone is a bout 100Kg according to the specs.
As for servo motors, personally wouldn't go near one. All my stuff is the clutch motor type. Good thing is parts avail for these kinds of machines is still quite good and the gears are metal.
Contemplated doing what you are doing to one of my smaller machines, but it never got past a thought.
Good luck with it