Thanks again, for yet another good video! Clear, and to the point.
I particularly like your "rant" about grounding, (but then I'm an electrician also).

Just my 2+2 cents:
a) -Are you having a GFI/ RCD on your shed's outlets as well?
Good thing to have before you'd need it, especially when rolling around outside with mains cables attached.

b) -Are there any RFI suppression capacitors/ A K A "condensers" built into these treadmill motors?
Also a good thing to have, as the back-EMF can be pretty nasty upstream at the rectifier/ SCR (see "d" below).
Should optimally be hooked from each brush to motor case, and one between the brushes for good measure.

c)-Have you tried checking your installations re: RFI?
Caps as per "b" will take 'em down, and an easy, cheap check is to put an AM radio
(tuned between stations) in the shed and listen before and after each improvement.

d) The ferrite cores found molded-in on old computer VGA cables or in electronics can come in handy for noise suppression-
just shove the phase & neutral leads thru it, as many turns as you can cram in, close to the SCR output.

I won't comment on your touching the ungrounded case in the test fire-up...

Cheers from DIYSwede (approximately at your antipode)