This may be an amateur question, as my area is Biophysics and not Cosmology, but what do you have to say about the Andromeda Galaxy which is at 2.5 million light years away and is moving towards us at about 70 miles per second? If gravity is repulsive beyond 1.5 million light years then how should we think about our closest major neighbor galaxy moving toward us? You may have addressed this somewhere but I don’t have time at the moment to go through your thoughtful post on reddit.
I would add though that this quote of yours from one of your replies on the Reddit thread struck me as interesting:
Someone please test this equation to see if slight adjustment of the constants will account for galactic motion or not. If it does, then proceed to the rest of the theory.
I can’t necessarily blame you that you want or perhaps need in this case for someone to test an equation for you and if so, then test the entire rest of your idea vs. all of the thousands of observations that could be tested. That’s a tall order for anyone to fill and is one of the challenges to theories of modified gravity - that is the large amount of success that General Relativity has along with many predictions of future data that we have indeed found/measured the most impressive being gravitational waves.