I strongly suspect the flex is because the spiral is kinked or bent. The piano wires are much much more senitive for damage than bowden wires. Ever so slight kink or sideshift of the spiral, and the wire will jam or get stiff. That's beacuse there is (and has to be) so small play btw. the wire and hole in the spiral. If you can withdraw the piano and feel along it with sensitive fingers, you will feel where the spiral has been kinked. A tip is to fix the spiral first, so you can use the wire as position reference, if you fix the wire first, you soon forget where the exact spiral damage is. Both the wire and spiral needs to be straight (or more likely, a smooth bow.) The spiral is very difficult to fix, but it is doable to an extent, with care. It's much same technique as correcting a spiral spring, if you tried that. On the bike, the length installation has to be so very precise, to not  have it too short or it can stretch and bend, or too long or it will kink at steering lock. Perfect spirals and wire, and the controls are wonderfully smooth and light without much flex at all.