I think you’re probably correct in that the mere existence of another dimension, or indeed multiple universes would not in itself prove the existence of God ) not of course, would it exclude it).
However, subject to one’s definition of God, I think the perception of these things outside of the 5 physical senses or rational mind does go some way to proving it.
As I said, I think much of this depends on your definition of God. I can’t really help in the context of some personal God, separate from us, because that’s not really my thing, my understanding of God is more influenced by Hermeticism and Neoplatonism (although maybe not 100% either). It’s difficult to explain, but let’s call God “The All” to avoid confusion with more personal Gods.
Everything Emanates from The All, everything that ever exists, has existed or will exist is part of The All (clues in the title). good and bad.Rather like the flame of a candle, it’s light bright closer to the candle lighting up the nothingness of a void, slowly, imperceptibly getting dimmer the further from The Source (the more common name in Neoplatonic thought).
Because we are part of The All we are part of everything, we can perceive all things in everything. We too are gods (this I think is Jesus). In Blake’s words “to see a world in a grain of sand”, in Jesus’s words, “the kingdom of god is within you”, lots of references to this thought process in both East and western philosophy.
Now who can see this? Who can walk between these worlds? Well I think we all potentially can, some much more easily than others, Aldous Huxley makes this point far more eloquently than I could ever hope to do in The Doors of Perception (obviously following the original concept from Blake. He laments his inability to naturally see these things as Blake, Swedenborg, Eckhart etc do as he takes his 4/10ths of a gram of mescaline (if you haven’t read this book I would very much recommend it, particularly with an edition that inc its sequel- Heaven and Hell.
Because we are all one everything reflects everything else. As above, So below as it says of the Emerald Green Tablet of Hermeticism or “on earth ad it is in Heaven” as it’s says in the Lord’s Prayer. Once you grasp the light of The All is everywhere, reflected and within all things you see the face of God, every second of every day and you can never un see it.
Sorry that’s completely rambling stuff and a v poor facsimile of my true understanding of its nature.