Unless the incursion zone is on the moon those planetary weapons are probably pretty useless. So that leaves them with the sphere, the anti-matter bombs which they admitted they might not be able to produce fast enough to keep up in the long run. And then there's Strange's power the team doesn't know of yet. And when they do find out, it'll probably be a last resort considering the cost.
But in the end, all that happened was that Banner put the pieces together by himself and Stark decided to make the best of the situation and try to recruit him. And honestly, who wouldn't be on board with this? The very first incursion showed a planet where Galactus was busy destroying it to prevent the death of the universe. Why, because he found out. Secrecy is vital.