A few points:
  1. Use the average of character-level attack values (the 5 in your frontline), not the team ATK value (which is not used by the game in damage calculations).
  2. Use grids of max level and skill level SRs and SSRs if you want to make this comparison relevant to endgame players. At least one of your SSRs isn't max level. You can calculate the change in attack value by simple addition/subtraction.
  3. Such calculations don't factor in one of the most important game mechanics: access to debuffs. Your best mono-element team will presumably have excellent debuff access; it's likely that at least one of your other elements will be debuff-deficient. You can use my Temporary Effects Calculator to estimate the effect of differential access to ATK and DEF down, as well as elemental advantage. The Advanced Damage Calculator can also factor in debuff miss rate (if you know this rate) and buff duration.