That would be the reason to wait until he’s out of office to impeach him - so he cannot pardon himself or anybody who aided him in his crimes, claim executive privilege to keep critical records hidden, etc.
It’s also why FEDERAL law enforcement would probably want to wait until he’s out of office to try him for crimes as well. Prosecution for state felonies would still work while he was in office because he has no control over state law enforcement. And he has committed perjury in Arizona and at least a couple other states, by submitting sworn statements saying he is Constitutionally qualified to be President, even though Hawaii has now confirmed that his Hawaii birth certificate is non-valid (which means he’s been using a different BC his whole life and knows darn well that he’s not eligible).
If he is out of office, you don't have to/can't impeach him. Because of politics, he is unlikely to face prosecution.