I don't know at exactly what point FO is better than your Rebels squad, because I think the Rebels probably are better (certainly on defense) at this point in the game.
That said, I did something similar to you and have some thoughts on FO at this stage.
First, max Kylo, Phasma and FOO. Those three are fairly easy farms, and work alright together. I ran these three at 7* G9 for a while along with a G9 STH and G9 IG-88. It worked alright, and kept me around Rank 50 on my shard. Note though that my shard is all my level, unlike you.
My team jumped a lot when I was able to replace IG-88 with Emperor Palpatine. He adds a lot of value to a "partial" FO team, providing healing and buff immunity (great against Nihilus leads, which I see a lot of) as well as stuns. He is definitely a positive replacement for FOTP until FOTP is at a similar level to your other characters. Mine isn't yet and Palp is hanging around until he is.
Because Trooper is only a bit ahead of FOTP for me, I used STH as his replacement. I recently got my Trooper to 5* G9, and I subbed him in for STH. Not a huge difference, but the counter is nice and I definitely notice a few more Assists under Phasma's lead. I don't think I would have used trooper before he hit 5* G9.
So currently I am running full FO, except I replace FOTP with Emperor Palp. I hit level 80 last night. What I just described has been my story for the past ten levels or so.
My team drops quite a bit over night, but that is the nature of First Order. I actually struggle beating Wiggs teams that move faster than me, but can beat ones that don't. I have FOO modded for Speed, and use Marching Orders on FOST if I am going to need to eat a lot of damage (against Wiggs) or on Palp if I need to prevent a Taunt (against a Sith Trooper, for example). I can climb with them. They're really fun. But they don't hold well on D.
Good luck! Hope some of this helps!