A few things - one, you don't have to use a team while you're building up its grid, just use light for now until your grid is leveled up.
Two, I'm not against healing, but for harder content, healing without any additional dmg mitigation and/or decent amount of ascension is usually inferior to a more offensive build. Healers buy you time while attackers kill things faster so you don't need to be healed at all. Heal works really well when combined with other dmg mitigation such as atk down and dmg cut, 'cos effectively each heal buys you a lot more time - this is analogous to how offense works better if you can debuff the enemies and/or buff yourself right before FB, so things die quickly. A lot of times though, I see ppl just randomly throwing in a healer into the mix without thinking about synergy, and that usually works out poorly.
Three, if you intend to play defensive, putting your primary healer and dmg mitigation in sub is a bad idea. Endurance builds work best when you have the endurance skills working starting at full health rather than when most of your team is mostly dead. E.g. if the enemy averages 2000 dmg/turn and you have 10k hp, you'll last 5 turns, if combined with atk down/dmg cut/heal, you cut the average dmg down to 1000 dmg/turn, you'll last 10T - buying you 5T. However, if you put your endurance himes in sub, and they come on when everyone else is at say 2k health, they only buy you 1T. If you go endurance route, you should generally have all your endurance himes in main not in sub. One exception is if you can control who dies first and transition from an offensive to more defensive build - e.g. sometimes I put Takeminakata in main and Sol AW in sub - I purposely FB before boss OD so Take is without swords and dies easily, then Sol comes on and heals up everyone else. These situations are pretty rare though.