You should first focus on getting valiant so you can have better rewards from daily super event 7* red skull and bastion looks to be the best candidates to rank 3
Definitely finish story content first. I’d do one run of every story quest that you can and then start exploring whatever the hardest story quest you can do is. (As a side note, the shooting stars (or whatever the 7* small content in the special quest tab is) is worth doing whenever you have enough 7*s. It’s easy and gives 7* shards). Truthfully, the everest content I would aim for once you finish story is a completion run of necropolis using the 7* chosen challengers Carina’s challenge. It’ll let you double up on necropolis completion rewards and the Carina’s challenge rewards. After that, a completion run of epoch of pain would be great if you have the 7*s to do it. If you can’t do that, I’d finish off the necropolis Carina’s challenges. The only Carina’s challenges that are worth it at this point are the necropolis ones and the ccp ones. This is all pretty far down the road though cause I’d crank out as much story content as possible before jumping into much harder Everest content. (Shooting stars is worth it though).