וַיֹּסֶף אַבְרָהָם וַיִּקַּח אִשָּׁה וּשְׁמָהּ קְטוּרָה׃
Abraham took another wife, whose name was Keturah. (Gen 25.1)
While the Gemmorah (Yevomos 62b) learns that a person must procreate through his whole life:
דתניא רבי יהושע אומר נשא אדם אשה בילדותו ישא אשה בזקנותו היו לו בנים בילדותו יהיו לו בנים בזקנותו ...
Rabbi Yehoshua says: If a man married a woman in his youth, and she passed away, he should marry another woman in his old age. If he had children in his youth, he should have more children in his old age, (This indicates that a man should continue having children even after he has fulfilled the mitzva to be fruitful and multiply).
I don't recall another example in our history of a man having children in his old age after fulfilling Pru Urvu Mitzvah, so I suspect that wasn't a real reason for Avraham.
So after Avraham has fulfilled his part by bringing up and educating Itzhak, why instead of learning Torah 24/7 did he put the effort in bringing five goyish children/nations?