All scripture is good for establishing doctrine.
This axiom is based on 2 Timothy 3:16-17 which says “All Scripture is God-breathed and profitable for teaching, for reproof, for correction, and for training in righteousness, that the person dedicated to God may be capable and equipped for every good work.”
Everything else that I write here is based on the assumption that this is a self-evident truth.
By taking it as a given that scripture can be used to understand scripture and form doctrine we bypass a lot of logical and doctrinal gymnastics attempting to get ourselves to the same point. Or, in other words, you have to trust something and I am happy to trust scripture.