In general, to balance the crossbowmen with the archers, only through the ability, according to my calculations, the radius of the armor-piercing attack needs to be... 8 tiles. With this setup, from 2 to 6 crossbowmen will deal approximately twice as much damage, from 7 to 8 exactly the same amount, and from 9 and above, more than twice as little. Naturally, no one would make such a large radius for the armor-piercing attack, so the difference is compensated by the reduced damage.
Regarding invisibility, I really don't like the TE's castration of the hunting hounds. Why remove the ability and add something new in the form of other abilities/stats when it can be fixed?
The mod is available for download (link in the first post of the thread).