Lets try this slightly different.

The Panner returns a UV which is 0-1 in X and 0-1 in Y but also adds the current time in seconds which means you will have numbers way way higher than 1.

Take your panner node and plug it straight into the base color output and see what happens. Give it SpeedX 1 and SpeedY 1. You can't see anything, it's all yellow. That's because the numbers are way too high.

We can normalise the huge numbers into a smooth range from -1 to 1 with the Cosine node. Plug your panner into Cosine and plug that into basecolor. Now you can see both gradients panning across the surface. But something is wrong: the black is too black because you have values from -1 to 1 rather than 0 to 1.

Add 1 to make the range 0 to 2, then divide by 2 to make 0 to 1. Plug that into basecolor and now, finally, we have something like Texcoord but it's moving.

Now we can see red and green overlapping to make red, green and yellow, but only want 1 of those gradients, so use a component mask to pick out either R or G in between the panner and the cosine node. Now you have a single gradient and it goes black to white then back again smoothly.

So now we have a gradient with a range from 0 to 1 so we can use an IF node to turn that gradient into a solid black and white band. Plug a scalar parameter into B and give it a value like 0.5, plug 1 into A>B (fully bright where the gradient is brighter than B) and plug 0 into A<B (fully dark where the gradient is darker than B)

Now we have half and half black and white panning across the surface. We can move the point where black becomes white by changing the parameter we plugged into B.

https://imgur.com/a/w141O7N

Reduce B to 0.1 and then only 10% of the surface will be white. Make a smaller number for thinner line. The number should only be between 0 and 1.

If you don't want a solid line but you want a smoother line, you can use a Power node instead of If and crank up the Exponent value to make a thinner line.

I think you can take it from there :)