You see New Zealand has developed massively in nearly two centuries
You say that as though the development has been wholly positive.
we have our own unique culture
Which has been built on a New Zealand specific concept of institutional bi-culturalism, meaning that Waitangi day is the perfect day to represent it. Moreover, the 'myth' or the 'story' of our 200 year history, and the beginning of our unique culture is heavily centred on Te Tiriti O Waitangi.
as it represents unity
From the article:
"Using the same pacifying rhetoric of “oneness” and “understanding” which has mythologised New Zealand’s somewhat chequered race-relations record, a ‘New Zealand Day’ would remove Waitangi – as a place and a founding document – from the veins of national identity. It would forcibly cleanse Māori influence on commemorations, therefore white-washing New Zealand’s bloody colonial past and doing away with need for constant apology."
But, nah, your 'New Zealand day' is totally different from the 'New Zealand day' discussed in the article. Your New Zealand day totally doesn't have implicit racist connotations. The writer of this article totally didn't see you coming a mile away. Let's just have a day where all the good little citizens who don't have problems with our current political, social, and legal frameworks can get together and fuckin' piss up m8, without all those pesky rabble raisers voicing their opinions!
diversity
You mean, institutional bi-culturalism which is built on Te Tiriti being our defining national document? Waitangi Day already celebrates diversity, it just does so in a way which acknowledges that in a diverse society not everyone thinks, believes, and acts the same! You want 'diversity' without the acknowledgement that our 'diverse' system disadvantages some groups for the benefit of others.
all round good times
Life in New Zealand isn't all round good times for everyone. Maybe it is for you, I don't know. But to have a holiday which celebrates 'all round' good times as our national holiday is just a way of ignoring the fact that for some people New Zealand is actually really fucking shit.
"Without Waitangi, our sense of nationhood is reduced to whimsical nonsense and shallow jingoism."
Seems that you prefer shallow jingoism to a forum for discussion of our history, present, and future. This article described your comment to a T.