So your employing the strategy of destroying the world before the Tyranids can harvest the biomass.
My issue is. How do you know what the Nids are up to or all about? You don't. You've never fought them before.
So fight to last breath but at first the Nids have the upper hand that's just how it works.
On the topic if making them change course you can do that, by offering them a larger meal. You can always attempt talking to them.