The time to challenge the legality of a subpoena is before
1) surrendering SOME documents
2) certifying that you’ve done the search and complied w/ the subpoena
I agree that the obstruction charge is the most compelling thing PDJT is facing. Depending on what evidence the DoJ has to back up their accusations in the indictment (and whether or not that evidence will be a) admissible in court and b) convincing to a jury) the obstruction charge could be the whole ball game.
The hope is Trump’s lawyers are up for this. The best thing is to appeal, appeal, delay. After election, this whole thing becomes mute win or lose.