So many genuinely anthemic story themes and illustrative techniques were introduced during that run, not just in FF (which were epic) but in Thor, X-men, NF/Shield, ... literally anything Kirby touched during that time. Conversely, so little was actually produced once he left Marvel, that you have to wonder how much Stan Lee was really involved except in broad outline.
Post Kirby, not much was going on outside of Spiderman, which was not terribly innovative once Ditko and Lee had established the character.
Dittos to all your thoughts. The greatest stretch were those years. Stan Lee was good indeed but given far more of the credit he deserved over Kirby imo. I have owned since those very years, 99%, (not important really anymore or it would have happened by now but someday I will get off my ancient can and buy FF # 43, ‘Lo, There Shall Be An Ending’, part 3 of the Frightful Four/FF trilogy, to complete my collection), of all of the Marvel issues from that era and would not part with ‘em for anything. (Some are worth quite a few $$$ as they are graded out in the mid nines).