Paul’s emphasis in Ephesians on Jesus’s kingship over all things, originating from His throne in heavenly places, makes sense in light of Paul’s status as a prisoner of and in Rome. The one true King is Jesus, and Caesar owes Him his allegiance. Psalms 2 and 110 loom in the background of the letter.
It’s worth noting how Paul continued working to take dominion for Christ, even though he was in prison (Phil. 4:22). The fact that Paul was Christ’s prisoner was more powerful than any claim Rome tried to lay on him (Eph.3:1, 4:1). And the same is true for all of us bound to one another in Him in the “bond (fetters) of peace” (4:3). Jesus ascended into the heavenly places and took captivity captive (4:8). Even though he was physically bound in Rome, Paul was still truly bound only to Christ, and thereby free from all others. No matter who we find ourselves “bound” to, Jesus alone is King, and we serve Him alone.
As a side note, I understand James Jordan’s idea of Ephesians 6 being about the High Priest, but I am not convinced by it. There’s no doubt that worship is warfare. But what is described in the passage is broader than corporate worship and applies to the whole Christian life. Paul’s emphasis in the letter is the working of Christ’s kingship first of all in the Church, but then also spreading beyond the walls of the local church building and beyond the relationships between members of the Church (chs. 5 & 6). What all believers are called to is spiritual warfare for Christ’s kingdom, everywhere and anywhere they go.
