Hi STW
I was in your exact position this time last year- i spent most of last year learning to surf at Muriwai and Piha so I even know your beach and waves well!
I got the same addiction you have and picked up a 7'6 (surf series) minimal within two weeks of my first lesson. I spent about 3 months on that before going to a 6'2 fish (which in hindsight was quite a big jump) but I perservered with it and got to know it. If anything, despite a year of surfing I am now wanting to go a bit longer and get a bit more volume rather than going for these super-low volume shortboards. Speaking from experience, the volume and length of the bigger boards is what you need when you're starting out to get as much actual 'wave time' as possible.
To make it more co-incidental, I am even the same weight as you!! Personally my favourite west Auckland break is 'maori bay', the little cove right next to muriwai. It performs on all tide times and is a bit more sheltered so the waves tend to be cleaner, easier to get out back and probably a bit more hollow + there's usually at least 2-3 different breaks to choose from.
I actually have a 7'0 minimal and an 8'0 minimal that I may be prepared to part with if you're interested. Could do you a good starting deal on both.
On another question- I know the conditions out west over the last couple of weeks have been perfect, clean and only 1-3ft but have you actually paddled all the way 'out the back' at piha to where the other shortboarders are sitting furthest out?
I've never seen a person on a softboard any further back than the whitewash and i'm not sure you understand how big a 7ft wave is, imagine a 4m high wollowing wave coming at you, or at worse, breaking on top of you! My mate lives in piha and surfs almost every day- an accomplished shortboarder. He said that even 5ft the other day was enough to smash him around and hold him under the water on the bigger sets. Keep the enjoyment and don't try to kill yourself prematurely!!

If you ever come out to muriwai or maori bay give me a message and i'll help you out + bring my boards for you to try.
Cheers