Branch circuit sizing for 5 HP 3-phase 230 V motor run on 208 V?
When selecting conductors for motors in the US, you do NOT use the motor nameplate FLA, you use values from a table in the NEC, in this case table 430.250. The value in that table for 5HP @208V is 16.7A, so you use 125%, ergo 20.9A. The 12ga 30A rating you quote is for 90C, you can only use that value when de-rating, so assuming you are correct on the de-rate factor (I'm not checking your work), you are still fine.

The only thing you use the nameplate FLA for is in selecting the Overload Protection.