I've been true f2p so far and have gotten to lvl 84.
Here are a few tips:
save daily crystals to refresh energy when farming annoying to farm characters (only one battle characters)
buy one of these characters from the squad arena store to get a jump on fleet battles: grand Moff, Ackbar, Windu(genrally considered bad by most players tho)
don't use the basic team
use energy resets: around lunch and dinner (not sure on exact times)
get good allies from the ally request thread (you still will need to gear and star your characters, but they can clear a few phases early game)
get journey guide characters