[1Co 15:28 KJV] 28 And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.
My prime suspect is Zechariah 14:9:
Zechariah 14:9 וְהָיָה יְהוָה לְמֶלֶךְ עַל־כָּל־הָאָרֶץ בַּיֹּום הַהוּא יִהְיֶה יְהוָה אֶחָד וּשְׁמֹו אֶחָֽד׃
The LXX seems to be formally similar:
Zechariah 14:9 καὶ ἔσται κύριος εἰς βασιλέα ἐπὶ πᾶσαν τὴν γῆν ἐν τῇ ἡμέρᾳ ἐκείνῃ ἔσται κύριος εἷς καὶ τὸ ὄνομα αὐτοῦ ἕν
Can one legitimately get from the Hebrew of Zechariah 14:9 to "that God may be all in all"?
Note: I have not heard him mention this verse that I can remember but I'm very influenced by Dr. Michael Heiser and his doctorate topic of Jewish binitarainism and "the second YHVH" idea which he discusses here: https://www.youtube.com/watch?v=pl3AMS6-BfQ