CAUTION: I never escaped the 9 speed era. If the following is nonsense, please ignore.
Could this symptom be brake shudder? It's relatively common in other disc brake systems. It is often a warped rotor but can also be caused by uneven pad wear. I have no idea if this is common in bike disc brake systems. I'm getting the impression that the OP tends to "ride the brakes" a lot. Perhaps quite hilly terrain? I don't understand how new pads and/or rotors would improve shimmy/wobble/head shake even briefly. OP: do you use your front brake a lot? Were the pads excessively or unevenly worn on replacement? Shimmy/wobble/head shake is a oscillatory movement around the steering axis inherent in all caster wheels. Shudder should be almost purely linear fore and aft. I also think it's probable a loose headset would have been found by the LBS or OP. But I've been wrong more times than I like to admit ;-)