The goal of flex is to reduce engine wear while accepting the potential risk of higher speed rejects. In other words, because you have less thrust it will take longer to get to V2 after engine failure and hence the higher V1. Higher V1 means potentially more hazardous stop.
Thus when using TOGA the calculated V1 is lower (plenty of thrust to get to V2 at 35/15 feet).
It is "acceptable" to use speeds based on flex and use TOGA but it defeats the purpose of flex in the first place
P