Took it all apart. Created a new screwdriver-bit-thing to turn the gearbox's input since that's the part that stripped from before.

Put it back together, now it knocks. Sounds like it has an engine in there. An upset engine.

I think the shafts (motor-gearbox) are (and have always been) misaligned. Either off angle, or off-center, (or both) not sure. I just never noticed because I was using hose clamps to soak up the vibration.

There's not really a way to get them lined up without throwing it all away. Just bad amateur machining on my part. It is how it is.

Really want it done, but really don't want to rebuild everything and keep bandaiding a sinking ship. Contemplating taking the chopsaw to the whole thing to permanently avoid the perpetual "one more weekend" delusion. Feel like someone sitting at a slot machine, can't walk away now, it's about to pay out! Nope.