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The apostles repeatedly say to read their letters and stick to what they said and wrote.

Which is fine advice! But they also say to hold fast to the traditions they taught, whether by letter or by word of mouth (2 Thess 2:15), and they say that the men they ordain have the authority and power to teach (2 Tm 2:2) and that they are to be obeyed (Heb 13:7, 17) and that Scripture can be hard to understand and can be easily misunderstood by the "ignorant and unstable" (2 Pt 3:16).

Sola scriptura fails to "stick fast" to that.

hold fast to the traditions they taught, whether by letter or by word of mouth (2 Thess 2:15)
Everything we need to know is recorded. They are long dead.
the men they ordain have the authority and power to teach (2 Tm 2:2) They are dead, killed and persecuted.
Scripture can be hard to understand and can be easily misunderstood by the "ignorant and unstable" (2 Pt 3:16).
That would be the willingly ignorant and unstable. Unless you are self-describing your own condition, the Scriptures were written for all to understand. You seem to be fragmenting and cherry picking Scripture to boost your bias which would be a perversion of Gods Word and purpose.

But [the apostles] also say to hold fast to the traditions they taught, whether by letter or by word of mouth (2 Thess 2:15),

I hold fast to the teachings of the apostles. I have their letters, given from them to the churches. Read in the churches since the beginning. This is a sure testament.

Why should I put faith in something that someone said the apostles said? If the letters of the apostles didn't contain everything that we needed to know, why didn't the early Christians write it down?

Please tell me something that I need to believe that isn't in the Bible that you believe the apostles taught.

and they say that the men they ordain have the authority and power to teach (2 Tm 2:2)

"And what you heard from me through many witnesses entrust to faithful people who will have the ability to teach others as well."

There is a big difference between ability and authority. For oral tradition, there is a restriction that many witnesses are required. A single person can not say that the apostles taught them something.

and that they are to be obeyed (Heb 13:7, 17)

Do you even read the verses you quote? Heb 13:7 7 Remember your leaders, those who spoke to you the word of God. Consider the outcome of their way of life, and imitate their faith.

How are we to know that they have spoken the word of God? The only sure word of God is from scripture. If I listen to a priest teach, the only word of God that he has spoken comes from scripture.

Heb 13:17, Obey your leaders and submit to them. Yes. Our leaders are those who speak the word of God(v7). But don't get the order confused. They do not speak the word of God because they are our leaders, but rather they are our leaders because they speak the word of God. We are servants of the word, not servants of our leader. If our leader does not speak the word of God, he is to be reproached and then removed if he doesn't repent. We do not owe mindless obedience to someone who it not speaking the word of God.

and that Scripture can be hard to understand and can be easily misunderstood by the "ignorant and unstable" (2 Pt 3:16).

Scripture is not hard to understand. Misquote much? There are some things(v16) in Paul's letters that are hard to understand. Because some things are hard to understand, have patience(v15), don't get carried away by error(v17), grow in grace and knowledge(v18). Nothing about mindless submission.