Good news about the blue (C) wire! Yeah, you could be using that now with your nest too - connect at C on the stat (Nest or T6 or any other modern stat), and connect the other end of the blue at C on the furnace board. You can connect multiple wires under those screws on the furnace board - typically there's 2 max (one on each side of the screw), but as long as they securely tightened down, you're golden. In this case, you'd have the blue wire and the existing red wire from AC connected at "C" - that's totally cool.
I kinda figured that 18-2 was probably going to either a pump or safety-switch -- that's just not a pump wiring setup I'm familiar with, but I know just enough HVAC stuff to be dangerous - I'm most definitely no expert!
Well, heck, I don't have a T6, but I sure think you'd be good-to-go with one, especially since you know you have a C wire readily available to use at both ends. I'm pretty sure you can just match your current nest wiring connection points on the T6 (and connect both ends of the C wire) and I think you should be good!
The other wiring (AC unit and pump) should be unaffected by switching out the stat using the same overall 1-for-1 wiring, but I'd be curious to hear if any T6 owners may be able to share any inside info.
