This may be after Sarah's death, but is not necessarily so; it is an account of other details of his history to complete it, and show the nations that were of his race.
1. " Then " is hardly a note of time; or rather, there is no " then "; it is " And Abraham added to take a wife." But Abraham lived 35 years after Isaac's marriage, how long Sarah had then been dead is not said—chapter 24: 67 would say not very long. The " added " is used for " again " or " another."
6. Most probably applies to a previous period—what, save as to their races, makes no part of his divine history. It looks as if Keturah was after Sarah's death, but " then again " is only English vay'ye-seph (and added); it is merely the additional fact, and Abraham took another wife, not positively saying when—though very possibly it may have been after the death of Sarah. He lived 75 years after Isaac's birth, being then too—Sarah died at 127, so that he lived towards 40 years after Sarah's death. Only Isaac's bringing Rebekah into his mother's tent, though much more meant to show the substitution of one to another, seems to hint that the time was not so long since her death. Abraham was 140 at Isaac's marriage, so that he lived 35 years after that; it was therefore, as I said, towards 40 years. Abraham was some 12 years older than Sarah, assuming her death and Isaac's marriage to be not far apart—a year or two.
7. Some thirty-four years after Isaac's marriage.
11. Elohim blesses Isaac. Here again it is God, as such—God's blessing on man.
21. The blessings for which Isaac entreats, and which are given are of Jehovah.
34. " He ate and drank, and rose up and went his way," refers, I think, to his profane indifference.