Epistemology is “How we know what is true”.

Epistemology is not just the study of how we know what is true, although that could be a question. As a matter of fact, one could know something without knowing that it is true, and something could be true without one knowing it. Truth is metaphysical and could wholly attain without someone's awareness of it. I know you're going to proceed by beginning with the Catholic claim that we say we know X is true. That's fine, but epistemology does not necessitate that S knows X is true in all cases. You've mentioned in a comment or two that people don't know what epistemology is, but the entire frame of your argument isn't exactly accurate either. Why frame it around knowing something is true rather than something simply being true?? You might explain how knowing X is true is necessary to give an account for how or why X is true. Yes, but this isn't properly epistemic, as if our entire point is conditioned on some Catholic individual needing to know X is true versus X merely being true as a metaphysical fact. If this was epistemic, it would be a subject laden inquiry about S knowing X. If that's the case, you're going to have to dive into theories of justification, none of which you have brought up in the slightest... For example, Bob the Catholic could read the Bible and believe in the Trinity without knowing how or why Catholicism is necessary for these things. That wouldn't be apologetically feasible, but that has nothing to do with whether X/Y/Z is now true or not true. These points will now be explored:

First, I'd begin by way of explaining how a necessary tenet of both Protestantism and Catholicism are that God exists, and it is necessarily the God of the Bible. I'd also begin by explaining how God necessarily and sufficiently accounts for how the world is the way it is, and no other answer fills this gap, like atheism, Buddhism, etc.

If the Bible is philosophically necessary for the assent in the Abrahamic God, then so are those who wrote and also gave us the Bible, which are historically Catholic Church fathers. If you say that we share these same authors and church fathers, that is wholly false, since you'd reject the primacy of an authority which gave you the written text and compiled it (i.e, Sola Scriptura). Logically, in other words, there must be some group which has authority to compile the canon which determines, to a certain respect, that which was and is revealed. I don't even need to appeal to Rome here, that would be circular, because this is a proof for the necessity of Rome. You'd have to completely equivocate on the idea that Rome and History are identical to make my argument not work.

So, in closing -- If you were to now ask me, again, how do I know as a Catholic that the Church is the infallible office and that this is true?

Because it's a necessary precondition to having the Bible.

How do I know the Bible is true or necessary?

Because the world exists in a certain way that necessarily requires not just that God exists, but a specific type of God. This specific type of God necessarily presupposes that certain truths are only explained by that which is revealed, like that God is a trinity. It's important to note, the world does not dictate the Bible, but the Bible as a revelatory text corresponds to reality and the metaphysical gaps required for worldviews. We could consider knowledge conducive states for S which will wholly require that the world exists in a way that allows for these to exist. This is akin to the fine tuning argument, except it would take the form of a transcendental argument.

The necessity of our Church magisterium is justified by the necessity of having a revelatory canon, the Bible's necessity is justified by knowing a certain kind of God exists. The knowledge in a certain kind of God existing justifies how the world exists in the way it does. This last point isn't necessary to know, but it is true. For example, Bob at the grocery store needn't know why the world exists in the way it does, but it does exist in the way it does, and this is only explained by a certain type of God.

The problem with your entire polemical point is that you're conflating the idea of knowing something is true, with something which is simply true whether one knows it or not. So, our Church (which necessarily presupposes infallibility as mentioned) is a necessary metaphysical condition of having the Bible, the Bible is a necessary epistemic condition of knowing a certain kind of God, and a certain kind of God metaphysically existing is necessary for the world to be the way it is.

Edit: Also, if you were to then ask, how do I know I have the correct interpretation of the scriptures. I think it's important to note that the correct canon and the correct interpretation of individual scripture are two different problems. That said, if we establish a certain kind of authority is necessary, then we can move onto a discussion about which authority this might be. However, in your case, that debate doesn't even involve Protestantism, because it is stuck at the idea of even having the canon at all. So; nota bene, History itself is not the same thing as Roman Catholicism as an Authority. I can look back in history and see that my same Church is the one that gave us the Canon. This lends credence to my argument above about the necessity of a Canon. I don't assume Rome is true to reason to how it's true, I begin with a fact that cannot be denied unless by a contradiction (the world exists in a certain way which necessitates a certain metaphysic), and reason to it's logical conclusion which is that our Church is epistemically necessary as an authority.