I agree with you that this is about Christ and the beginning of a new creation. He will eventually fill the whole universe with Himself. It starts in His new body, the ecclesia. This is the first that is raised from out of the dead. It is holy and flawless in God sight and God is now free to tabernacle fully in Christ's body as well which is the ecclesia.
(This is probably what it means to obtain the glory of Christ.)
God chose you as the firstfruits for salvation, through sanctification by the Spirit and through belief in the truth, to which he called you through our gospel, so that you may obtain the glory of our Lord Jesus Christ. (2 Thessalonians 2:13–14)
In other words, this tabernacle has now been made ready for God's Spirit through Christ to fully dwell in the body of Christ, the same way it dwells in Christ now. The ecclesia is now considered Christ's body, the complement or fullness of Christ. He has been made complete with this body who has been raised up with Him seated in the Heavenly places.
Literal Standard Version:
until we may all come to the unity of faith and of the recognition of the Son of God, to a perfect man, to a measure of stature of the fullness of the Christ,
which is His body, the fullness of Him who fills all in all.
World English Bible:
which is his body, the fullness of him who fills all in all.
Young's Literal Translation:
which is his body, the fulness of Him who is filling the all in all,
18And himself is the head of the body — the assembly — who is a beginning, a first-born out of the dead, that he might become in all [things] — himself — first, 19because in him it did please all the fulness to tabernacle,
He will fill all things with Himself and this is just the beginning.
The final phrase "that He might fill all things" includes the sense of completion (Colossians 1:18–19;)
Weymouth New Testament:
He who descended is the same as He who ascended again far above all the Heavens in order to fill the universe.)
World English Bible:
He who descended is the one who also ascended far above all the heavens, that he might fill all things.
Young's Literal Translation:
he who went down is the same also who went up far above all the heavens, that He may fill all things -- Ephesians 4:10
Since we are being built for habitation for God, He will eventually fill all with Himself.
God may be all in all.
I submit to you for your consideration that the pleroma, fullness is actually Christ's complement that is being referred to in this verse.
Someone said there's 78 verses between Ephesians and Colossians that are similar since they both have to do with Christ and the ecclesia which is His body, His complement.
"Both words come from the Latin complēre (meaning "to complete"). Complement refers to something that completes something else, while compliment has branched off to mean a remark "especially in the form of admiration." Webster dictionary
It refers to His body, Where is Christ is its head. It is Christ's complement.
In conclusion there is a new creation of Christ that has come into existence, but has not yet been revealed in glory. This πλήρωμα, plērōma, often translated fullness, had also been translated as the complement.
fullness, a filling up
Usage: (a) a fill, fullness; full complement; supply, patch, supplement, (b) fullness, filling, fulfillment, completion.
The ecclesia is holy and flawless in Christ in itself so to speak. The body of Christ is His complement, the same way Christ is God's complement.
Here are a few pieces from an article that explains the fullness or complement of Christ.
THE ECCLESIA AS CHRIST'S COMPLEMENT
by John H. Essex
...the ecclesia, which is His body, the complement by which He is completing the all in all (Eph.1:22,23).
The word "body" is here used for the first time in this epistle, and is connected with the word "ecclesia." It will be used again on several occasions later in this letter, and it is similarly used about the same number of times in the Colossian epistle. For example, "...and He is the Head of the body, the ecclesia" (Col.1:18).
Our Lord, we know, created all. Without Him, nothing came into being that has come into being (John 1:3; Col.1:15,16). And yet, among all the celestial inhabitants of space, there was not one who could be found to be His complement. For His complement, too, must come from within, and that is why we find ourselves, if we are members of that ecclesia which is His body, "chosen in Him before the disruption of the world" (Eph.1:4).
Just as woman, as man's complement, maintains the form of humanity throughout all generations, so the ecclesia, as Christ's complement, maintains the form of the ministry which God has begun in Christ, so that God shall find "glory in the ecclesia, and in Christ Jesus, for all the generations of the eon of the eons. Amen!" (Eph.3:21). THE ECCLESIA AS CHRIST'S COMPLEMENT by John H. Essex.
And He is the Head of the body, the ecclesia, Who is Sovereign, Firstborn from among the dead, that in all He may becoming first, for in Him The entire complement delights to to dwell, Ephesians 1:18-19 Concordant Literal