I've run many fights to test the compos you guys gave me, and I came to these conclusions:
There are much better choices than Chirrut alone, without Baze. Concerning Raid Han, he's an amazing toon, but he doesn't benefits/gives enough of the Jyn's lead. CHan, imho, is very good in TB but is not an arena character. Chopper has a nice kit, with a good synergy concerning the protection recovery, but he really only shines in a phoenix squad where he can share it's unique buff.
With a zJyn lead, OB is indeed a beast.Thanks to the 35% potency he deals TONS of debuffs => tons of exposes. When he dies he gives TONS of buffs => tons of %protection + 50%health/protec. He is not so fast but he also gains 5% tm each time a rebel is attacked, so he's not so slow either. So:
zJyn
OB
R2 is still mandatory, without its stealth CLS can easily bypass taunt and destroy zJyn.
For the 4th role, I hesitate between CLS, Leia, whose zeta synergises well with zJyn lead's and who benefits greatly of the stealth to remain out of harm, and with ATF. I have yet to 7* her and see how it works out, if she can duplicate enough buffs to hit hard enough
For the 5th role, I might use a DPS support, eg Cassian when the ennemy team does not have 2 or 3 cleansers, or a tank support, such as Zeb or Kanan.
Anyway, trying to build a team outside the meta (even though still rebel :D ) is not that simple so thank you for the help, it's really appreciated. And any idea/constructive criticism is still welcomed! :)