Dash, on its own, isn’t too strong, I think, in its current iteration. Will Power recovery from kills isn’t enough (unless you’re dealing with extremely fragile alien configs) to allow you to cycle dashing and killing endlessly. In combination with other abilities (and a shotgun!) it is definitely way too strong (at least at certain stages of the game - further in than I am it might be a necessary evil). Another alternative solution to the unending kill chain could be lowering the will recovery from kill.
At present, if you can’t kill an alien in one attack, you’re losing on Action Points and Will Points (unless the alien returns 4 or more will) OR if the alien doesn’t refund more than 1 Will Point you’re slowly losing Will Points.
At present, many tankier Triton and Arthron configurations can’t be killed in one attack by any weapon short of a Hel Cannon or a Shotgun. If you didn’t have a shotgun and were faced with these tankier variants using decent weaponry, 15-20 at a time, the skill combo would be strong but not necessarily seem OP and could possibly even be necessary to have a fighting chance. Weaker variants able to be dispatched by even 2 handgun shots while offering 2-3 will points per kill really highlight how OP the combo is.
How many things are combined in making the combo OP really make it hard for me to agree to where the change needs to come to fix it. Of your proposed solutions, though…
Only being able to Dash OR use other willpower only skills would make most skills worthless in many situations, make Will Points as a resource much harder to use and is not a solution I like the sound of at all.
The other two are definitely avenues I would agree are worth exploring.