Hold on there. You've conflated a number of things here.

> Anselm doesn't really need to prove the existence of God.

I cannot comprehend this statement.

> Christians don't need to prove it; they have a Bible that they take as fundamental revelation.

Why trust the Bible? Its authority must rest on extra-biblical knowledge (in Catholicism called Tradition). The Catholic Church compiled the text in light of Tradition. It didn't fall from the sky. But regardless, the point is that knowledge of the existence of God is not a matter of revelation. Indeed, the Catechism of the Catholic Church (a kind of summary of the beliefs of the Church, not a didactic manual or comprehensive text) teaches as much [0]:

Our holy mother, the Church, holds and teaches that God, the first principle and last end of all things, can be known with certainty from the created world by the natural light of human reason. [...] Without this capacity, man would not be able to welcome God's revelation. Man has this capacity because he is created "in the image of God".

If you're going to write a check like that, you better be ready to cash it! And there are many ways to cash it (like these, to name a few [1]).

> They know that this argument isn't going to convert anybody on its own.

While many people aren't persuaded by proofs about most things, this does not make proofs worthless. First, those who are moved by proofs will benefit. Second, proofs can lead to greater clarity of what we mean by "God". This is because in order to understand a proof, one often needs to discuss presuppositions and ancillary propositions to appreciate it. You are also given a point of departure for further discussion that can deepen your grasp of the subject matter. Third, those who aren't moved by proofs can still benefit from some reassurance that such proofs do, in fact, exist. This is still true for those who do not know or understand them, just as one need not have a comprehension of medicine to take comfort in the claims of one's doctor.

> The "better-than-all-other-things" definition is meaningless and doesn't tell us anything we want to know about God.

You're confusing the question of whether God exists with what God is like. Why? Knowing about God is certainly desirable, but to establish God's existence through reason alone is a worthy enough exercise. I don't understand the compulsion to dismiss the value of establishing existence. It's not either/or, it's both. Existence and essence. (Of course, we come to understand that God's essence just is existence: Ipsum Esse Subsistens.)

> Nor is it Biblical; it doesn't prove anything at all about the Bronze-Age-Middle-East deity Anselm believed in.

I have no idea how you arrived at this claim. How is it not Biblical? This frankly sounds like some strange 20th century concoction of Protestantism. Reason has always been central to Christianity, certainly in the Catholic tradition. This is just basic knowledge ("fides et ratio"). As reason is what is the most essential and definitive characteristic of human beings, and the basis for understanding anything, it would be absurd to assent to any faith that opposes it. Not only that, but the very first verse of the Gospel of John, for all intents and purposes, equates Christ with Reason itself (the Incarnate Logos). The God we're talking about here is not some mere "Bronze-Age-Middle-East deity", one being among many, one ubermensch among many pagan alternatives, but Being Itself.

(Also, here's a take on Anselm [2] that you may find will clarify things.)

[0] https://www.vatican.va/archive/ENG0015/__PB.HTM

[1] https://a.co/d/7SnyEST

[2] https://edwardfeser.blogspot.com/2010/11/anselms-ontological...