Oh gosh there's a lot to catch you up on. I'll try to be succinct.

For general unit worth, ffbe wiki has them ranked. 5* base units (rainbow crystals) are quite powerful, namely orlandeau, noctis, gilgamesh, olive, all being physical damage dealers. Especially since orlandeaus banner is now, increasing his summon rates, I would reroll (make new accounts) until you get an orlandeau, then stick with that account - especially if you've only played 2 days. He has the best potential to chain, or build a damage multiplier for crazy damage, of any unit in the game thus far. Not sure on how much money you spent, but if your best units are the ones listed, I would recommend a reroll in any case :/. There are many rerolling guides online. Chizuru and luka are decent, but not enough so to be the stars of your team. To give you perspective, I'm rank 56 and have 6* max luka, ling, CoD, chizuru, noctis, refia, yshtola, exdeath. And my main party is noctis, yshtola, chizuru, ling, exdeath/CoD depending. Just got a cecil so maxing him now and he'll take exdeath/cod slot. So you're striving for a team as good or a little better than that by 2-3 months of play time (I'm f2p and medium lucky). If you're planning to be casual and not strive for maxing a lineup and getting TMRs, then don't reroll, but to be competitive and clear content effectively, top tier characters are not essential, but very helpful. Also there are a couple awesome tmrs from certain characters like gilgamesh and zidane, so getting units like those is good to look for too, but not as essential as 5* base. TMR rankings are on ffbe wiki too.