I cannot use the Inspector at work, so... does anyone know if a 5 * Non-VI weapon or a 3*++ VI weapon will do more damage in an FFVI stage?
I can tell you the formula to figure it out yourself if you're a mathy person.
Basically look up the stats for each item here:
https://ffrkstrategy.gamematome.jp/game/951/wiki/Home
You need specifically the base atk and max atk.
For the 3* weapon, the max level fully combined is 20, and for the 5* weapon the max level fully combined is 30.
Let T1 and T2 be the base and max atk value of the 3* weapon. Calculate the value LT = (T2-T1)/19. This is the level up factor for the 3* weapon (e.g. how much the stat increases each time you level).
Let F1 and F2 be the base and max atk value of the 5* weapon. Calculate the value LF = (F2-F1)/29. This is the level up factor of your 5* weapon.
All synergy does is give you a certain number of bonus levels. The formula for this is bonus_levels = 20 + 10 * (Floor(current_level/5) - 1).
So for your level 20 3++ weapon, the number of bonus levels is 20 + 10 * 3 = 50. This means the item has an effective level of 20 + 50 = 70.
Now, to compute the atk of the 3* VI weapon with synergy, just compute base_atk + LT*(70-1)
Example:
Dark Sword (IV) (5*)
Base Atk: 77
Max Atk: 128
Bastard Sword (VI) (3*)
Base Atk: 23
Max Atk: 54
Bastard Sword Calculation:
LT = (54 - 23)/(20 - 1) = 1.6316
Effective Level with Synergy = 70
Atk value with synergy: 23 + (70-1)*1.6316 = 136
Dark Sword Calculation:
LT = (128-77) / (30 - 1) = 1.7586
Maximum level at 5* (no synergy) = 20
Atk value at level 20 = 77 + 1.7586*20 = 113
So in this case the Bastard Sword would be better.