So, in watching the movie for the umpteenth time I finally made the effort to decode the fireworks from the scene in Sarah's apartment that we all know to be coded. Like many other people here surely do, I assumed it was Morse code. It's hard to really make it out, but I was able to get a few letters: •- / ••• / -•-• / -• / -•• which is A/S/C/N/D. So, that almost spells "ascend" right? I was pretty excited.
A post* I found after my happy revelation did a better job than I did (having found "I ASCEND NOW") but, I think we can confirm now that the fireworks do in fact say "ASCEND" in Morse code.
*(https://www.reddit.com/r/underthesilverlake/comments/a8h6d4/the_fireworks/?utm_medium=andr