For that strategy:Frankly, the way I see it.
"If they shoot first, we're already in a shooting war, and just meekly accepting defeat isn't the way to go"
They've regressed heavily, and have mostly denied any resupply, and weapons they use on a decoy facility is weapons they don't have for the subsequent attack--and if they had the ability to solo the entire planet, they wouldn't be playing fuckfuck games with WMDs the way they are.
I'm not saying "We need to spam enough defense emplacements to stop them firing every nuke on the planet at us suddenly", but we should spend enough that they can't take us down with anything but blowing their whole stockpile, which forks them pretty damn hard.
The Mechanicus are strong, but their influence from what we've seen comes down to "We're willing to shoot first and we have first strike capability", not the sort of mass and advanced armaments they need to win a straight fight, which is why they're doing this WMD chicken thing. "We can't beat you conventionally, so we'll make sure that opposing us breaks you too." and all that. In that sense, the trick is going to be setting the bar that they can't just "Accidentally" fire a cruise missile at us and take out our decoy facility, but need to straight up take the mask off and either bombard us with most of their arsenal--which means they don't have it later, and just blew it on our decoy facility--or launch a ground attack, which plays to our strengths, since we've got more dudes than they do most likely.
And if we lose the aboveground facility? But cost them their first strike ability and capacity to repeat the feat in short order? We just won.
Because if they had the power to take the entire planet on, they'd just be in charge right now.
Would you include a free action to estimate the Mechanicus production capability for nukes?
To know if its "purely old stockpile", "make extremely solely"(~turns per nuke), "makes solely"(~nuke per turn) etc?