In the ad intra relations of the Godhead, the Father is first, unbegotten. The Son is second, begotten. The Spirit third, proceeding. The order (taxis) is always the Father first, the Son second, and the Spirit third, and never reversed or switched. Obviously, the persons are fully equal and fully God, so what does this order mean?
The order of relations is an expression that the Father is always the Father, the Son is always the Son, and the Spirit is always the Spirit. It is not as if three undifferentiated persons in eternity past came together and deliberated which person will be the Father, which person will be the Son and which person will be the Spirit, perhaps through a divine coin toss! No, the persons are who they are eternally and have never and will never change. Consequently, we speak of God as "the Father, the Son and the Holy Spirit." Ever wonder why we do not say "the Son, the Father, and the Holy Spirit" or "the Holy Spirit, the Father and the Son" or other such permutations? We say "the Father, the Son, and the Holy Spirit" because the Father is first in relation, the Son is second in relation, and the Spirit is third in relation. Every time we insist on the formula "the Father, the Son, and the Holy Spirit," we are insisting on this order of relation. If anyone denies this order of relations, then they should have no problems with saying "the Son, the Father and the Holy Spirit" (the ultimate "Christocentric" phrase), or "the Holy Spirit, the Son and the Father" for after all, the persons qua order of persons is fully arbitrary and could very well be otherwise.
Of course, to say that the order of persons could be otherwise attacks the doctrine of immutability. I am not that sure it would compromise the doctrine of aseity since I do not subscribe to Thomistic (Aristotelian) metaphysis. But compromising the doctrine of immutability should be bad enough. Egalitarians who think of the Trinity as a "divine dance" of three persons who are as good as interchangeable are in error at this point. The Son is always the Son, and thus this is reflected in His workings ad extra as an eternal submission to the Father. The Son always submits to the Father, and to say that it is only restricted to the incarnation is a failure to deal with the entirety of the text of Scripture. Particularly in the Reformed tradition, the pactum salutis was made in eternity past. It cannot be timeless but in "time" since the pactum is a "past event" at least before the incarnation. To claim that the pactum is "eternal" in the idea of "timelessness" is to create a time paradox whereby the making of the pactum has a "temporal" overlap with the incarnation and the crucifixion (regardless of whether one takes "timeless" as outside time or as all-time-in-an-instance, or sempiternal). If one takes "timeless" as "outside time," then the making of the pactum is a stative event relative to the incarnation and the crucifixion, so the making of the pactum IS at the time of the crucifixion (static "action"). If one takes "timeless" as sempiternal, then the pactum is in the eternal PROCESS of being made at the time of the incarnation and the crucifixion (perpetual dynamic action). Both scenarios should be unacceptable for Christian theism.
Since the Pactum is past in relation to the incarnation and the crucifixion, and yet eternal, it must be in eternity past, the eternity in the sense of "everlasting time." While yes, we can say that the Pactum has its origin ad intra, because the plan of God comes from within the being of God, yet it is made ad extra in everlasting "time." It must be so otherwise we would have the crazy time paradoxes which makes nonsense of the entire plan of God.
God has an immutable order of relations of the persons in His being. As such, this is reflected in the ad extra eternal submission of the Son in the Pactum Salutis, which is not "timeless" or "sempiternal" but in the past in eternity (everlasting). To say otherwise would be to compromise something either of God or of God's plan, and thus to go contrary to Scripture.