But we are way ahead now. We know that it is assembled correctly and we have the collective pitch adjusted close.
Now, you can start looking for the exact number of shims to get rid of the "nod".
I know this is a pain, but there isn't any other way to do it.
You might need 2 more shims, or 2 less, usually nodding is cause by too tight damping, that is too many shims, but it can also be from too loose.
Since you were getting results from turning the washer over, that seems to me that you need tighter damping (I know I guessed opposite at first). So put in one more shim per side, and fly it and see. It will either get better, or it will get worse!

We had to get the washers in right to make sure we werent getting that sticky collective.
It is just barely possible that you got another bad servo, too, did you get the same s75s that were in it originally, I have had really bad luck with those servos, I have seen them go bad right out of the box.
If you suspect this you can swap one servo for another and see if the nod goes with the servo. But still the damping is the most likely cause at this point I think.

By the way after I posted yesterday, mine started nodding too! I think it is contagious.
But mine was caused by the tail rotor not pushing exactly horizontal, and the gyro trying to correct for it.
If your tail bracket got knocked loosi in the crash, it might be crooked, like mine was, and evidently this will cause nodding too, as I found out yesterday.

Another thing is the blades too tight in the grips, this can cause all kinds of problems.

Sorry to seem so haphazard, I have a visitor from out of town and I'm a bit distracted, so I might not be posting until tomorrow, so I wanted to mention as many possibilities as I could think of.