The reason you should accept that The Ultimate Reality is God is because this is how the concept is understood in theology. Should the scholars of geology define the language used by mathematicians? Of course not.
First off, theology does not have a monolithic definition of god. Secondly, I don't recognize theologians as experts in reality, truth, or whatever else you might be defining god as, so, an appeal to their authority here is faulty.
The Ultimate Reality is That which is Ultimately Real. The realest reality. The Truest Reality.
We already have a label for that: reality. I have no problem accepting reality exist. However, calling it "god" doesn't add anything to it and I wonder why we would do this. Let's say we were to accept reality and god were the same thing...then what? Do we just call reality "god" instead of "reality"? What edification does this achieve?
It is true by itself, it is not contingent on anything else to be true. It is eternally true, never not true. It is The Truth itself.
You're asserting reality is not contingent and eternal, but...we don't know anything beyond the beginning of our own universe. You can have your own opinions, but you can't have your own facts! Additionally, it makes no sense to call reality 'truth'. Is a tree true? Are clouds false? Reality is not true or false, it just 'what is' and true and/or false are not applicable terms. So, whether conflating god and reality makes sense, you're justifications for doing so fall short.