Yeah, the Rebellion was formally declared after the
Ghorman Massacre, but even in R1 years later, Jyn is dismissive about the concept of a Rebel "Alliance" of the various cells and movements. Things don't get rolling until the one-two punch of Scarif and Yavin, and even then, they're still largely decentralized, with the de facto "main cell" transitioning from being the Massaii group on Yavin to the Skywalker group on Hoth (for all of ten minutes).
That's something that seems to have changed from the old EU, with the Rebellion having been a lot more "regular" for most of its existence, where the new canon puts a lot more emphasis in Rebels, R1, and Andor about how the Rebellion is a movement, not a state, and that there are, what was the phrase, whole battalions who don't even realize they're part of an army against the Empire yet.
Something I'd like to see more of, actually, are the
large variety of starfighter liveries seen in the Squadrons video games make it into canon. We've got the Partisans, but most of the other ships we see seem to use the standard OT markings, even deep into the New Republic era (and including, weirdly, the pre-Rebellion Y-Wing on Ferrix in season 1, which had the standard Gold Squadron markings like it had just flown in from Yavin).