but don't most or all of the charges in his business fraud case pre- date his presidency?

It is hard to keep up, but yes, much of the activity pre-dates his presidency. Not all, though. I think some of the payments (and therefore the fraudulent recording of them), happened after he was sworn in. Of course, those are all personal in nature and therefore outside of immunity. But that won’t stop Trump from claiming it is immune and appealing all the way to his friends in the USSC.

There’s also testimony from Hope Hicks (and maybe a couple of others) that might not be admissible. Hicks was a White House employee, so her conversations with Trump might not be usable in court. That one has a better chance of working to Trump’s benefit. What I don’t know is how important that testimony is to the overall case. If it means a couple charges need to be dropped, I suspect the DA will let those go. If it’s the whole case, that’s a fight for the DA to take on. (Much like Jack Smith is keeping Pence in his Jan 6 case but dropping all other White House staff.)

—Peter