Almost all cards technically disallow (in their T&Cs) earning points for business transactions, but almost none of the card issuers seem to enforce this. AFAIK, Woolworths is just about the only card issuer who have actually been known to cancel cards if they suspect business spend is going through them. So, you should just try to figure which card(s) will earn you max points for min cost and go for those, bearing in mind there's always a small risk of the card issuer eventually catching on and cancelling the card.
On that last point, with over $800k/year of spend I'd suggest the Citibank Select card (as you've already been exploring) will probably be the best "value" option for you assuming you're happy with KrisFlyer / Velocity / AsiaMiles points. I've personally put hundreds of thousands of dollars of business tax through my Select in the past few months without issue, and have also earned full points even when I've started with the card ~$100k in credit. I know others on here have had the same experience too.
Ironically, I checked the CitiBusiness Gold card's T&Cs a few months back and they say business transactions are not allowed