In Ezra 7:21, Artaxerxes refers to God as the “God of heaven.” Is he thinking in polytheistic terms here (I.e. God as head of pantheon or a Baal-like deity)?
The title, "God of Heaven" occurs in numerous places in the Bible in all three Bible languages as follows:
Hebrew: Ps 136:26, Gen 24:3, 7; 2 Chron 36:23; Ezra 1:2; Neh 1:4, 5, 2:4, 20; Jonah 1:9.
Aramaic: Ezra 5:12, 6:9, 10, 7:12, 21, 23; Dan 2:18, 19, 37, 44.
Greek: Rev 11:13, 16:11.
Further, the title, "God of Heaven" is very likely an abbreviation of "the LORD, the God of Heaven and God of Earth" from its earliest occurrence (Gen 24:3); and thus is not unique to the Persian rulers. However, it was used more often in Persian and Babylonian times than all others in the Bible.
Note that "God of heaven" was not unique to Hebrew theology. The same title occurs (with slight variations) in polytheistic theologies as well. See https://en.wikipedia.org/wiki/Sky_deity
However, "God of Heaven" appears to be the most common name of the Jews' One God when talking to the Persians.