“Your argument is situational. That we should carve out exceptions and go outside the proper Constitutional remedy because, oh dear, this or that crime is so heinous.”
You’re simply assuming that the Constitution prohibits criminal prosecution of a president for anything done in office (official duty or not) unless and until there’s an impeachment and conviction. I don’t see the text of the Constitution as supporting that claim.
Thinking about horrible things a president might do isn’t situational. It’s part of understanding what the Framers meant. Did they mean that a president who committed treason, took bribes, or murdered someone on Fifth Avenue couldn’t be prosecuted unless the whole cumbersome mechanism of impeachment and conviction occurred first? That interpretation would have the potential to produce ridiculous results, and that potential is a reason for concluding that the Framers probably didn’t intend that.
There’s also the more obvious point that if they had meant it, they could have said it.
Article II section 4 would be superfluous if the head of the executive branch and the nation’s chief magistrate could be subject to prosecution under Article III. The beautify of the Constitution is its simplicity.