Shield stability could be weakened when a shield is reflecting damage (weapon does not penetrate).
Let’s say 1 “stability point” per reflected hit above 20 net damage (after modifier). This way massive multi-warhead missile spam does not help (they do not enough damage), but hitting a Shield tank massively with more than one higher-damage weapon will at a point in time make the shields fail temporarily.
Very interesting idea. Although the issue with shield stability is that it is all or nothing, so you would really needs to bombard it like crazy to actually make it fail, or are you thinking that it acts as a companion mechanic to maybe having some, but not enough shield stability weaponry?
My idea was along your second thinking, have more weapons be able to weaken the shield stabilizers (with less effect), so I may also be able to achieve corner cases by using standard weaponry and not only the specialist weapon.
Also, it is something I would think of as being “rational” or “normal” - maybe it could also be depending not only the net shield damage of the weapon but also the difference between the shield penetration value of the weapon and the actual resistance value of the shield (the lower the difference, the higher the effect on the shield stability).