The Venus flyby does indeed make the mission shorter, but it has some pretty serious negative consequences as well.
The mission spends much more time in deep space.
Approaching the sun will increase the amount of radiation exposure by a large amount.
The thermal design would need to be rethought to survive that close to the Sun.
The time on Mars for a Venus ...
This is an interesting question in that it highlights what gravitational assists can and can not do.
To think about this, first choose a frame, then find something moving fast in that frame, then come at it and let it gravitationally accelerate you for a few hours roughly in the direction of its motion.
For example if the frame is our solar ...