Well, if it is one of your favorite plants, then you should really give it a bigger home for the growing root mass.
Sure, you could cut off some of the roots, but that means first, shock to the plant, two, that much less surface area for the plant to take in water, nutrients, and some other functions roots do.
The plant will respond to the imbalace caused by your part by losing some leaves or try and grow more roots to compensate for the loss.
You mentioned all the roots are healthy. The best thing to do for the plant is to move it to a larger pot.
That's just how it is with phals. You could chop down cattleyas and oncidiums, but phals, well, you just have to give it bigger pot.
Again, cutting the roots are only temporary option as the plant WILL grow roots again, very soon!
Then, again, it is your plant.
Cut, let the wounds dry or apply sulfur powder, then put it back to the same pot.