This passage is a chiastic synonymous parallelism
The chiasmus takes the form A-B-B'-A'
A. God is my salvation
B. He will keep me safe
2. Synonymous parallelism
As Victor Ludlow has observed:
Parallelism is the most distinctive quality of Hebrew poetry...In
parallelism, a thought, idea, grammar pattern, or key word of the
first line is repeated or continued in the second line. There are two
basic types of parallelism, grammatical and semantic...Among the types
of semantic parallelism...Synonymous parallelism: a theme of the first
line repeats itself in the second line, but in slightly different
words (Isaiah: Prophet, Seer, And Poet p. 32)
In order to avoid repeating the words of the first line (God is my salvation) verbatim, the idea is repeated in slightly different words. It's worth noting that the "is" in "God is my salvation" is not actually present, but implied: אֵ֧ל יְשׁוּעָתִ֛י
Along the same lines, the parallelism also uses different forms of the word salvation in the first and second repetitions: יְשׁוּעָתִ֛י vs. לִֽישׁוּעָֽה׃. (Also of interest--the root here for salvation is Strong's 3444, "yeshuah")
It appears that Isaiah's principal motivation in varying the words is to be poetic--he does this a lot.
This passage and the surrounding verses have often been taken as a millennial prophecy--see especially the preceding thoughts in Isaiah 11: 6-10. Verse 10 in particular is worth citing:
And in that day there shall be a root of Jesse, which shall stand for
an ensign of the people; to it shall the Gentiles seek: and his rest
shall be glorious. (Isaiah 11:10)
On this interpretation "that day" refers to the millennium (to be sure, this is applying New Testament ideas to Old Testament prophecy), in which the Lord will provide His people with a rest that will be glorious. So in that sense Isaiah can acknowledge that God is his salvation, but in a coming day that salvation will be more fully realized (Isaiah 12:1 specifies "in that day").