How do I explain it? The ship with Twilight of Equestria is super dead, Sci-Twi was not interested in him, and Sunset …. I really don’t know how they haven’t talked about this, but going back to her makes it look like a third option, in my city it’s called a third table dish, the Flashimmer can be considered:
“Oh well, I didn’t get anything with Twilight, so I’ll try to flirt with her version of this world, oh dammit she didn’t want anything with me either, oh my ex is being nice to me, I’ll try to get back to her after being rejected 2 times in a row”
And that makes it a very bad ship, it makes Flash look bad, because he seems like someone desperate for a love relationship who will change girl if the other did not want him, we would have to restart that, giving him a ship with some secondary character other than Twilight or Sunset, the opportunity to do a good job with those ships has been lost, trying to fix that is like fighting an elephant using a stick.
Sorry dude, but fix Flash is no longer possible, when they wanted to improve the character in the shorts it was too late, 4 movies where he did nothing more than a cliche, several specials where he appeared in the background without any dialogue, a short singing a song, and some interactions with Sunset that did not go beyond greetings and outings from friends, which lasted like 6 or 8 seconds in screen, when the end of MLP season 9 was approaching, they are not enough to improve the character, and make a ship based on that, would be forced, trying it no longer had a case, I like what you propose, but fixing a character with 5 years being bad character, it would have been unnatural even if they had given time.