Yes, its longer than the opening (because the post was not in line with the opening and I didn’t want it to swing through the other way), yes, the post is not level, yes, I should probably not have used an existing fence post, no, I will not be fixing it. But look at the fit on the diagonal. MINT
When the diagonal is mounted snugly like here, the only way for the door to sag, would require the diagonal getting shorter. You are not going to push a solid piece of wood shorter like that. If the diagonal was the other way up, as is often seen, screws would be needed to try to keep it from pulling apart. However, that would not last.