The term "the artifice of eternity" seems difficult to understand, but in prosaic language Yeats seems to be saying that he expects the elements that compose his mind and body to be reabsorbed into the total material that makes up nature. When this happens, the material that formerly made him will be mixed with other matter and gradually turned into different forms, being shaped and absorbed, reshaped and reabsorbed, through the transforming power that keeps perpetually changing everything throughout eternity. What he is thinking sounds similar to what Shelley says about Keats in "Adonais":
He is made one with Nature: there is heard
His voice in all her music.
Yeats seems to go a step further, however. He expects to be absorbed into Nature and then transformed into something that is "out of Nature." This would appear to be only temporary, because nothing at all can last throughout eternity. There will be infinite transformations. The first one Yeats is wishing for is to find himself included in
...such a form as Grecian goldsmiths make
Of hammered gold and gold enameling
That would last much longer than a weak human body such as he is inhabiting now. It would give him thousands of years to focus on his poetry and perhaps enable him to reach the heights that are unattainable for him now.