So the rate up right now isn't really worth it? Still trying to figure out what units are really worth it and which are garbage. Also I'm trying to finish up the quest line and it seems a bit strange that you can basically auto through a dungeon until the final boss and then depending on your team comp or friends available it just feels like a brick wall.
The meta game is currently going through a bit of upheaval with all the new seven stars, and new content which is phasing out one hit ko squads.
The most important thing to have is a mitigator. You have darvanshel, shera and Narza which covers everything. Shera has the highest drop checks and offensive buff, darv has lower dc and a defensive buff. So shera is your normal mitigator for questing and FH, darv is better when facing earth enemies, and Narza slots in when you are fighting bosses that have BB gauge drain. So you are good on that.
After mitigators, the next important role is status null. Rigness, Ulkina, and Ahvel cover that with the first two having the added bonus of heals. Kanon and Ronel 7* cover this aspect as leaders, also adding in a nice hp buff. (you want to shoot for a combo of kanon+feeva or ronel+elimo)
Defensive buffs will become more important soon which is why Kanon is such a popular leader in japan.
After that you have BC/HC drop buffs: Feeva, ronel and zelnite are all awesome for that,
Then you have damage buffs: crit and spark are the most beneficial, attack% sucks in comparison. Rossetta, Feng, and Elza all are awesome spark buffers (rosetta has the best leader skill) but all are going to get outclassed as leaders by and upcoming 7* and as subs by Alfa dilith (free unit from trial 6)