Pony POV Series, Siren World · 9:19am Aug 12th, 2023
To note, in the hypothetical reality where the sirens used Nightmare Eclipse's time reset spell to make their ideal history and ruled an interstellar empire, (with entire planets dedicated to nothing but producing negativity to feed them), with their two lost sisters at their side, and Equus' civilization having been guided by their hoof for 5000+ years, (no age of Lost Dreams, no Age of Discord, no Age of the Two Sisters, No Age of the One Sister, etc) ...
After the sirens spent a 1000 years bringing all of Equus civilization under their control, the technological progress would be insanely fast, quickly becoming equals of other space fairing civilizations.
Not just ponies, but the other sapient species on Equus being their thralls (including newborn sapient species like the griffins).
The sirens would have been able to eventually manifest Morning Star on the physical plane, becoming one of their super weapons in expanding their empire (though the wars of resistance against them ALSO feed them negativity, so it was a win-win for them), Morning Star APPROVING of the Sirens' world, where mortals 'know their place' on the Sirens' ruled worlds (entire worlds of unthinking sycophants), and the negativity farm worlds 'show the family the mortals' true nature.' Possibly granting them MORE power and making them an EVEN BIGGER threat to the pony universe at large. Becoming the soulless puppeteer world Morning Star wanted all along.
Again, the concepts of reality from all across the spectrum would likely incarnate on different planets to stem the tide... Killing Tiamat would be a high priority... so she could reincarnate in another mortal life and fight at their side free of the enslavement of five sirens enthralling all five of her mortal brains! (to note, the sirens would become the first entities to ever accomplish this, and even they have to pool collective efforts of ALL FIVE OF THEM fueled by entire planets producing nothing but negativity to keep Tiamat in line).
Tiamat would likely reincarnate as a Radaptor, evolving four extra heads. Or more likely, five Radaptors incarnate, then merge into one five headed Tiamat instead of five separate identities.
https://www.deviantart.com/night-chimeras-cry/art/Alien-Sketches-730449821
Of course, all of the worlds you've written are hypothetical at best. Such is the nature of fiction.