The problem is: no, they absolutely would not. His tax information is going to be most damning in relation to other data - e.g. showing his involvement in money laundering, which isn’t even a crime that the IRS would be looking at. So it’s not just about what tax laws he violated, but I also reject your assumptions. Hell, we already know, from newspaper reporting, that Trump engaged in all sorts of tax and bank fraud over the years that didn’t get caught. (But the reported instances are too far back to be prosecuted now.) The IRS has always been terrible at detecting the malfeasance of the wealthy and even more so in recent years as they’ve been de-funded and moved their attention to lower-income filings.
Really? [expletive deleted] seriously? Documents relative to accusations that Trump engaging in more recent instances of the kinds of crimes we know he’s historically engaged in is “a lot” like a totally baseless, racist conspiracy theory that refused to accept reality? (Personally I’d say the two situations are complete fucking inverses of each other, not similar.)
To be clear - a legal demand has been made. What Trump or his lawyer have to say about it, whatever objections they have, has fuck all to do with the process - they’re irrelevant. They’re not being asked for, nor providing the documents. If a Trump appointee in the IRS decides to break the law, there is a process for dealing with it, and it should - must - be dealt with, because that, in itself, is a fucking scandal.
The problem is that we know the Mueller report was only investigating one, specific issue: Russian interference in the election. It’s quite likely that Trump simply passively accepted Russian help, and, separately, sold out US foreign policy (e.g. getting the Republican platform changed to be more pro-Russia) to the Russians because he does - hoped to do - business there, which would be outside the scope of the investigation. It doesn’t even cover all the multitude of crimes and ethics violations Trump has committed since the election. This administration is so broadly corrupt and criminal, it would be absurd to focus on one specific issue and ignore the rest. We need more investigations just to begin to cover all the things Trump has done openly.