@Gay
Thank you very much for your insights! They are very helpful. Your tip of raising the presser foot slightly with the knee lifter is really interesting. I saw someone doing that when sewing decorative trims onto a wedding dress. She would sew straight for a second and then raise the presser foot with her knee and free motion the larger bits of the trim onto the dress, then back to straight stitching for the narrower parts.
I'm guessing that you had a clutch motor in your machine. Those make noise all the time, even when not sewing. The servo motors are more like domestic machine motors, don't run constantly and are very quiet. A modern Juki industrial is very quiet, easy to control and really robust.
Removing feed dogs is a matter of removing the stitching plate and un-screwing one screw. I haven't done it, but have seen many videos of it being done. Then when replacing the plate, you might as well put in a darning plate. But it would take two minutes. Then I would add a quilting foot and raise the presser foot pressure. Raising the presser foot pressure is something you probably have to do even with the Juki semi-industrials.
When I'm quilting on a domestic, I feel like I'm going to break it. I want a robust sit-down machine that I can also quilt on. The noise the semi-industrials make at higher speeds is actually a migraine trigger for me, which is a little of why I'm looking at something quieter. I test drove a Juki semi-industrial and a Juki true industrial side-by-side, and the true industrial is far quieter and smoother.