I definitely remember the fic being RariTwi hence my posting here. Details I remember include Rarity and Twilight having been dating for a while, but are pretending they're going on their first date (or something similar sounding), Cadence and Shining are there near the end of the fic alluding to the ending, and the ending where Twilight proposes to Rarity.

"Simulations" by Monochromatic sounds like what you're looking for

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Exactly what I was going to say.

Here's the link - https://www.fimfiction.net/story/460363/simulations

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Ah looks like I forgot to favorite it the first time I read it. Thanks a bunch for helping.