The Person of Jesus Christ, the God-Man, is one of the Great Mysteries in the Bible. Eternally Yahweh and EQUAL to The Father and Holy Spirit. I am not too sure you can press 1 Corinthians 15:28 to show the "subjection" of Jesus to the Father post-Ascension, which John 17:5 rules out.
You admitt, "I am not too sure you can press 1 Corinthians 15:28 . . . ."
1 Corinthians 15:25-26, "For he must reign, till he hath put all enemies under his feet.
The last enemy
that shall be destroyed
is death. . . ." That happens in Revelation 20:14.
So 1 Corinthians 15:27-28, ". . . when all things shall be subdued unto him, . . ." understandably (to me) takes place
after Judgment day (Revelation 20:14) when Christ receives His bride from Heaven on to the New Earth, when He as the Son begins His actual
eternal rule (Isaiah 9:6-7, Luke 1:32). Revelation 21:1, Isaiah 65:17-18.
John 17:5 begain with HIm being
again at the right hand of God, and
how this was, was only what changed. John 3:13 KJV, Hebrews 1:3, John 1:2.