You're misunderstanding me.
First, I see Trump as a business person, not a politician. In business, if they want to fire you they'll often wait until you 'step in it' in a public way and then fire you. Comey 'stepped in it' in his Congressional testimony. So, Trump acted then. Just my guess, but I think that's why the timming.
I watched a fair deal of Comey's testimony, I personally think his testimony is what got him into trouble. And I don't mean the slip about "hundreds and thousands of emails". His explanation for why he felt he needed to usurp the AG's role and make the announcement was rather outrageous. And his whole 'looking back I wouldn't have done anything differently' is straight up outrageous. The boob learned nothing. If looking back he can't see that he made mistakes he's utterly and helplessly incompetent. Anyone with two brain cells would have said that, yeah, it was mistake to break with FBI policy and announce Hillary's investigation (twice, mind you).
Fern