Al Capone wasn’t prosecuted for tax evasion because it’s cool or smart or strategic to prosecute murderers for lesser crimes, but because proving murder in court was going to be more difficult.
That’s not analogous to a Congressional impeachment, but the opposite of how Congress operates. Congress sits on indisputable evidence of the greatest crimes while impeaching presidents for lesser offenses that are harder to prove.
Andrew Johnson publicly did everything he could to limit “freedom”