What does the first "all refer too in "the fullness of Him who fills all in all", Ephesians 1:23
Here is a different version that sheds light on the "all" that is referred too.
And subjects all under His feet, and gives Him the headship over all to the ecclesia which is His body, the complement which is completing the entire universe. Ephesians 1: 22-23
The fullness of Him is His body, known as the complement or pleroma.
"The sovereignty of the Earth is given to the nation of Israel, according to the prophet's. The sovereignty of the heavens is the portion of the ecclesia which is His body. Between the two the entire universe is brought under the administration of Christ. Thus His body is the pleroma, or complement, which fills up the lack which earth's deliverance would still leave in the celestial realms. Its function is to fulfill God's purpose for the entire universe, only part of what He is able to accomplish through his people, Israel." Concordant, literal commentary.
As Eve came out of Adam and completed Man to rule over the earth, so the ecclesia that was chosen before the disruption or foundation of the world, (Eph.1:4) makes Him complete with His body.
God seated Him at His right
that is seated with Him above every sovereignty and authority, power, and dominion, and every name that is named, not only in this eon, but in that which is future also: and subjects all under His feet, and gives Him the headship over all to the to the ecclesia which is His body, the complement which is completing, the entire universe.
It's also stated in Colossians.
And He is the head of the body, the ecclesia, Who is Sovereign, Firstborn from among the dead, that is all He may be first, seeing the entire complement delights to dwell in Him..,
The body being seated with Him is also confirmed in Colossians 3:1
Not only have, we died and been raised with Christ, but we have ascended into heaven with Him.
If, then then, you were roused together with Christ, be seeking that which is above, where Christ is sitting at the right hand of God.
Christ is Head of the ecclesia, and He is the Savior of the body. Nevertheless, as the ecclesia is subject to Christ...
This all goes together what is stated in Ephesians 1:10
He purposed in Him, to have an administration, the complement of the eras, in which the universe is to be headed up in the Christ--
So now, with all that said, the first all is the ecclesia, which is His body. He is the head.