Once out of nature I shall never take My bodily form from any natural thing, (25-26)
The speaker’s given nature a try. Now it’s time to move on. Art, he thinks, doesn’t have to deal with the same fears of aging and decay that living creatures do. Maybe being a painting isn’t so bad, after all.
and louder sing For every tatter in its mortal dress, (11-12)
Bemoaning the fact that folks seem to give up as soon as they grow older, the speaker angrily reflects that "souls" should sing all the more strongly once their bodies start to deteriorate. For one thing, they’ve got a lot more to say now than they did years and years ago. For another, they don’t have much time left.