Apollonius wrote:For what it is worth, I think Keats is thinking about Shakespeare here.
You cannot find Shakespeare the man in his plays. You find all of humanity. Of Shakespeare himslef, his politics, his religion, his tastes, there is nothing. The poet is gone and disappeared behind his work. Only the poetry remains.
I think Keats does this best in To Autumn.
Yes...and of course Keats so admired and loved Shakespeare for his singular ability to separate himself from the psychological truths illustrated in his characters rather than reflected on him. But there is a bit of irony here with regard to his philosophies of the Camelion Poet...Keats integrates himself into his work
constantly...unbeknownst to him...this is one the important reasons he is revered (alongside Shakespeare at that!) and widely studied--as an epitomized example of Romanticism as purveyed through individual feeling...
The annihilation of the poet is the identity "dissolving" as it were. Keats used that idea repeatedly--the "dissloving and forgetting" or the "ceasing to be." There is a definitive bridge there between his feelings of death and poetry...(commingling love in there, as well.) This idea helps reinforce his thoughts on holy truth within imagination (the poet's)--or that capability of "being in uncertainties"--there again, a Romantic notion.
I agree with you about "To Autumn." If there was to be an exemplary showcasing of his "becoming the thing itself" that would be it--I think critics agree, as they often dub that ode as "the most perfect poem in the English language."
Perhaps some of his longer narrative poems possess this quality to an extent, too--like
Lamia,
Eve of St. Agnes...but even in those, you find hints of 'characters' or people who were large forces in his actual life--especially in his female characters.
The fact that he was indeed that close to his work and seemed to have reached those heights he longed for through poetry makes his epitaph all the more
achingly sad, doesn't it?

"I am certain of nothing but the holiness of the heart's affections and the Truth of Imagination."