If you object about why anything would be added to the Creed on the basis that doing so is illegetimate, it’s not incorrect to point out that the same was done at Constantinople, and that the canon about not adding to the Nicene Creed (and not the Nicene-Constantinople Creed) is to be understood as prohibiting adding anything contrary to the Nicene Faith, for if that wasn’t the case, the creed professed at Constantinople would not make any sense.
When the Advocate comes, whom I will send to you from the Father—the Spirit of truth who goes out from the Father—he will testify about me. (John 15:26)
Nevertheless I tell you the truth: it is to your advantage that I go away, for if I go not away, the Counselor will not come to you; but if I go, I will send him to you (John 16:7)
Being therefore exalted at the right hand of God, and having received from the Father the promise of the Holy Spirit, he has poured out this that you yourselves are seeing and hearing. (Acts 2:33)
References to the Spirit of the Son: Rom 8:9, Gal 4:6, Phil 1:19, 1 Pt 1:11
And there is numerous Patristic evidence against the idea that the Spirit proceeds from the Father alone, which is what (at least some) Orthodox confess.
The Catholics, like the Orthodox, confess God as the Unbegotten. He is the First Principle, in a sense, of the Trinity. Part of the objections between east and west ultimately boil down to differences between Latin and Greek on the words for procedes, cause, and principle. The words in Latin allow for it, but it becomes nonsensical or heterodox when fitted into the Greek terms.
Do you reject the notion that the Father has given all things to the Son? That the Son is not the perfect image of the Father? Do you claim that Spiration is unique to Paternity alone?