In terms of strengthening your fire team, Mars would actually be the better choice imo. Yamaraja already covers the B-frames well enough and with Mars debuff + Ambush, you'd hit the def down cap. Res down can also help you to land other debuffs more reliably if you're not playing mordred. Amaterasu would be a stronger pick if you also wanted to use her in other teams, like for example dark.
EDIT: Do you really need to strengthen your light team further currently? Your team is pretty much the same as mine, just with Raphael instead of Michael and with that you can cruise through any current content. Michael would just add more build diversity (at least until awakening). I'd personally rather focus on key members for teams that aren't somewhat complete yet. Amaterasu would prob be my top pick for your setup currently, but if you intend to roll on cybele, it's prob best to just wait and decide afterwards.