Right, there are other verses than this that talk of God being the “Rock” of our Salvation, and the only one etc.
Of course it’s true that He is the source of all Salvation, and in this sense, He is this “Rock”. From Him all Salvation comes. That is what is meant here. Also, of course it means that there are none that are equal to, or “beside” Him.
There is no contradiction in believing that, and also believing that Simon was called “a rock” as well, a rock that Jesus built His Church upon. This notion is also in OT Scripture.
Isa 51:1-2, this describes Abraham as being the “rock from which you were hewn”.
Note that same word for “rock” in the Hebrew is the same word used in your reference (Isa 44:8).
That same word (H6697) is also used in Jer 21:13, where God speaks of “being against” the “rock” in the valley. Is God against Himself here? Of course not.
The point is it all depends on context, and how the word “rock” is used in which passage. In Jer 21:13 it’s used as just a natural rock, in Isa 44:8 it’s used to describe God and the source of salvation, in Isa 51:1-2 it’s used to describe Abraham as the source (earthly) of all Israelites, in a fleshly way.
This is all about the Hebrew word for “rock”. In the NT there’s no difference in this only in the language used (Greek). In the NT, in reference to Simon, the word for “rock” is used in reference to him, but of course it’s always understood that the only way he is “rock” is because of Jesus, because without Jesus Simon is nothing.
Just as Abraham is nothing without God. Just as the “rocks” of the valley are nothing compared to their Creator. All are subject to Him who is the “Rock of Salvation”, no other “rock” who is “beside” Him.
That’s just understood. That’s a given. And thus should be understood anytime anyone (any Catholic) refers to Simon Peter as “rock”. Simon is only “rock” relative to (subservient to) God, he is a “rock” from which God made him a “rock”, but God is the source of all “rock” as He is the source of everything. Simon is not a “rock” equal to God in other words, so Isa 44:8, and similar such verses that speak of God being a “rock”, isn’t violated here.