The more I think about it, the more I think Luxon's lack of political experience is a problem for National. And perhaps their advisors aren't so good either.

Ruling out working with Opportunity based on one policy he doesn't like shows a complete lack of understanding of how a multi-party government works. Say you won't agree to the land tax but will try to find common ground in another area?

I get he was trying to stop National voters from going for Opportunity in the hope of a National/Opportunity party, but it still seemed like the wrong call.

Yeah Opportunity fall under the Radical centrism umbrella, which that wikipedia page actually has them listed. It's basically what you said, they have big ideas (radical) but are very pragmatic, will work with anyone (centrism) to move things in the right direction. This is opposed to the idealists Greens or Act which stand on their side so don't get as many opportunities to progress any of their policies.