And yet, even here Paul does not say that "God appointed Himself as Son".
Paul maintained a distinction between God the Father and Jesus his Son.
So I don't see a problem with it.
Did you skip over that verse 8 where God says:
But about the Son he says, Your throne, O God, will last for ever and ever; a scepter of justice will be the scepter of your kingdom.
God calling the Son, God. Right there. Hard to miss or ignore.