Seeing as Liana is being kept in gacha our next free healer in rarity is Chydis who is also an elf,so the fact that she is plat,is an elf(meaning higher atk spd),and her skill is the same as spica meaning better range and healing she is pretty good.Chydis isn't really replacable unless you have B.Iris,and Liana even then though the faster atk spd means she isn't healing at the same pace as other healers.Also think of Chydis for a new player standpoint you only have Iris,and the silver healers unless you get lucky in the gacha and pull Fedora,Camillia,B.Iris,or Liana.Now thats just for Chydis.
You also have to think about Rachi who for a long time is actually gonna be the only free Mage Armor till Erica,who btw is looking to be a pretty bad unit in the first place.
Then we have gold Pirate Murette who her sub could be skipped and she just be added to tp since the only thing her sub did was detrimine her rarity,which is good she gold for lower Demon crystals,and less resources to lvl up.
The Plat Princess(forgot her name) also falls into Murette's situation as well without tp rotation.
Theres also gold Avenger Anubis(I think thats his name),and the Gold Dragonrider.Though I don't use em so no exp with em.
Constant TP rotation is pretty vital,and I'm sorta biased towards then rotating since I never got Chydis cause I never was able to get enough crystals in time.