The Fire meta team is Hyperreal, Nanamech and Empyrea. Anything other than that is basically copium. If you go on Gray Ravens' page, you'll even see that from the moment that Nanamech releases, she immediately replaces Pulse on every single team comp, even the weakest one (which would be Ember/Nanamech/Empyrea).

Unfortunately, all the upcoming S ranks are meta for their respective teams, so regardless of who you choose to skip, you'll miss out. You CAN use the new units with older ones, but it's a disadvantage.

I'm not a very experienced player but this is what I would say (totally my opinion, just making it clear). Prioritize the Attackers. Yes, you need Tanks and Amps to ideally reach a good point, but let's be real, any team without a proper Attacker is not in a good spot. Especially when the Attackers we're getting are actually good. You can get away with a very well built Attacker having mediocre supports, but for the supports to be at the level of the attackers they have to be quite something, and many of them would probably require quite the investment.