I don’t think the fundamental purpose of this passage is a discussion of chronology.
I see it as a call to repent and obey the first commandment.
I readily concede that more than one meaning may be intended by this passage—this is a common feature of Isaiah (see “Isaiah Prophet, Seer, and Poet” pp. 53-54)—and there have been a number of thoughtful explanations here. But the principal interpretation I get out of this passage hasn’t been mentioned, so I’ll throw in two cents.
--
Before me (לְפָנַי֙ “lepanay”)
לְפָנַי֙ is used dozens of times in the Old Testament to mean “before me” in the sense of “in my presence”
(see here). A few examples include Genesis 40:9 & Nehemiah 6:19. There need not be any chronology explicit in the term.
--
After (וְאַחֲרַ֖י “weaharay”)
This comes from the word אַחַר “achar”, forms of which are used for “after”, “followed”, “succeeded” (as in one king succeeded another see 1 Chronicles 27:34), and notably, in Jeremiah 7:6 “neither walk after other gods”. (see here)
--
The 1st commandment
Isaiah 43:10 contains an echo of the first commandment.
Thou shalt have no other gods before me. (Exodus 20:3)
A literal rendering of the Hebrew in Exodus 20:3 would be
“Not|shall-you-have|to-yourselves|gods|other|before|the-face-[of-me]”
(see Sola Gratia’s explanation on this site here)
Thus Isaiah appears to be echoing God’s command in Exodus “don’t have any other gods in my presence.” The context of the surrounding verses further suggest that the first commandment is the central focus here.
--
Context from Chapter 42—who is worshipped?
In the previous chapter we are reminded to give the glory to the God of Israel:
Let them give glory unto the Lord, and declare his praise in the
islands. (Isaiah 42:12)
And not to worship false gods:
They shall be turned back, they shall be greatly ashamed, that trust
in graven images, that say to the molten images, Ye are our gods.
(Isaiah 42:10)
--
Context from Chapter 43—The God of Israel presents His resume
In the early portion of chapter 43 God speaks of what He done for His people.
Immediately after the verse in question further context is given—the God of Israel is presenting his resume and commanding them to worship Him and nobody else:
11 I, even I, am the Lord; and beside me there is no saviour.
12 I have declared, and have saved, and I have shewed, when there was
no strange god among you: therefore ye are my witnesses, saith the
Lord, that I am God.
…
15 I am the Lord, your Holy One, the creator of Israel, your King.
…
25 I, even I, am he that blotteth out thy transgressions for mine own
sake, and will not remember thy sins.
--
What is the point?
The point made in the subsequent verses is to call His people to repentance so He can justly forgive:
22 But thou hast not called upon me, O Jacob; but thou hast been weary
of me, O Israel.
23 Thou hast not brought me the small cattle of thy burnt offerings;
neither hast thou honoured me with thy sacrifices. I have not caused
thee to serve with an offering, nor wearied thee with incense.
24 Thou hast bought me no sweet cane with money, neither hast thou
filled me with the fat of thy sacrifices: but thou hast made me to
serve with thy sins, thou hast wearied me with thine iniquities.
25 I, even I, am he that blotteth out thy transgressions for mine own
sake, and will not remember thy sins.
26 Put me in remembrance: let us plead together: declare thou, that
thou mayest be justified.
--
Priorities
The context suggests to me that the purpose of this passage is not a lesson in chronology, but a call to repentance. “Thou shalt have no other gods before me” is as much about leaving behind competing priorities (you can’t bring them into His presence) as bowing down to idols.
--
“Before me there was no God formed”
The God of Israel commands that He is to be the number one priority in their lives, and that certainly nothing made by human hands should displace devotion to God. Not only does He pre-exist any human invention, but He should occupy a place in their hearts that nothing formed by human hands can.
--
“Neither shall there be after me”
Nothing devised by humans will ever change the above, and it isn’t enough to worship the God of Israel and something else. His people are to worship the God of Israel and none other. While the Savior can justify people and enable them to enter God’s presence (compare Isaiah 6:1-8), people can never justify their own creations and bring those creations into God’s presence.
As clever as people are, they will never create something that replaces their need for their God, nor will they find an alternate route into His presence.