can i get banned for buying quatz?

No, you won't get banned. It's a very common thing to do.

You're fine. This is how people in the EU play FGO to begin with

You will be fine.

I'm from NZ (FGO is also not available here and I have to download the game via apk too). I've been playing and buying quartz for over 3 months and they have gladly taken my money

No, it's fine. But the only important thing is if you want to buy a google play gift card, buy the one with the same currency of your server. I bought IDR gift card because my region changed the currency of the SQ pack, only to find out that i can only use USD gift card.