I have experience with local sports funding and in journalism, and I think some of the accusations being made here are quite unfair.
First of all, there is nothing to suggest that the timing is strategic other than it being published in the weekend paper, which is when investigative sports stories are normally run. Just because the sources talk about meetings from a couple of months ago doesn't mean he or Stuff have been waiting for some ~best~ time to hurt the club. We know that journalists have been complaining about Welnix not getting back to them, Liam probably couldn't get them on the record about what he'd found until this week.
Why hasn't he looked at other teams/sports? My guess is that, as pointed out in the article, he decided to dig into the Phoenix after the response he received to a question relating to the foundation. Could he have turned this into a big exposé about every sport under the sun? Sure, but he was intrigued about a specific club in a fairly unique situation. He has mentioned on Twitter that he might take a look at rugby funding next, though that is something that has been reported on quite a few times in the past, and is even mentioned in the article itself.
As for why no-one has investigated ACFC? That's because NZF have determined that the Prem is amateur and there is a standard contract for players that is all about "training fees" and "match fees" that it has been decided does not contravene their amateur status because it's just a proxy for reimbursement for travel and gear expenses. The method of paying players for coaching is a pretty well-established method of paying high-level amateur players, which is why how they were paying kids like Joel Stevens was okay.