Leveling up Characters

I already have 11 heros that are maxed out at 7*, so your saying I should go for gear 12 1st

At G11 FO can solo the whole of the pit raid T7.
As can many other teams.

If arena is focus, get them up first and foremost.
If pit is focus, look at teams that can solo it and G11 them.

FO with decent speed can solo it much earlier than G11 if it's the right team:

KRU, FOO, FOE, FOST/Phasma/Kylo (pick two). First three are needed and can even solo it on their own.

Above G10 you should be able to solo without the Rancor or pigs taking any turns at all.

Benefits are: no legendaries required (but get you a legendary instead), capable at low gear, very little reliance on RNG, almost impossible to screw it up, and incredibly fast on auto once modded and geared up (8 ish minutes). Oh, and they can almost solo P2 of the HAAT themselves.

What is FO

And the other question would be. What is the best mod to use, or do mods really make much difference?

First Order

Oh, I am mostly trying to complete the events and raids, not so much, the arena

Is 1st order the best heros

My 5 highest power heros are Darth VaderG9, Mace Windu G8, Ahsoks Tano G8, Luke Skywalker Farmboy G8 and Jedi Consular, G7

Not even close, although they make a good TW/GA defense team. Best is the latest legendary releases as it is apparent CG is trying to get a big amt of money