You’ll have to read German for this one but it looks interesting. “Isaac and Ishmael: The relationship of the two sons of Abraham according to Genesis 17 and Galatians 4.21-31.”
Jakob Wöhrle, “Isaak und Ismael: zum Verhältnis der beiden Abrahamsöhne nach Genesis 17 und Galater 4,21-31.” Evangelische Theologie. 71:2 (2011): 115-132. HT
For those of you only conversant in English but are interested in the Ishmael angle in Galatians you can check out the following:
Borgen, Peder. “Some Hebrew and Pagan Features in Philo’s and Paul’s Interpretation of Hagar and Ishmael.” In, New Testament and Hellenistic Judaism. Edited by Peder Borgen and Soren Giversen. Aarhus: Aarhus University Press, 1995. 151-164.
Bruce, F. F. “‘Abraham Had Two Sons’ A Study in Pauline Hermeneutics.” In, New Testament Studies: Essays in Honor of Ray Summers in His Sixty-Fifth Years. Ed. Huber L. Drumwright and Curits Vaughan. Waco: Markham Press Fund, 1975. 71-84.
Thomas, Kenneth J. “Covenant in Relation to Hagar and Ishmael in Galatians.” Bible Translator. 37:4 (1986): 445-446.