Hey! I'm always glad to see people engage with the issue, regardless if you agree.

We can both agree that all ought be tested against Scripture, but where we might not is if anything outside of Scripture must be believed.

In the Gospel of John (21:25), it is written that not all of what Christ did and said were written down, for it would fill the libraries of the world. This definitely leaves room for things to which Christ intended for us to hold fast but were yet not written down explicitly (2 Thess. 2:15, "...either by word of mouth or by our letter").

Some things remained oral after the Apostolic Age, and this would include things like the nature of Christ (one person in two natures, begotten not made, consubstantial with the Father, etc.), the Trinity explicitly stated (three persons in one being), etc.

Holy Scripture is the highest but not the only authoritative element of Holy Tradition.