A venus flyby would be similar to Apollo 10 where they would go to another body, and then do a free return fly-by, maybe with engine assist, and return back in a year. Regardless of if/when we go to mars, there will be at least one "free return" mission before they attempt future missions that have a "parking orbit" around mars.
Going to Venus would allow us to simulate a mars free return trip, but in only 1 year (which would mean for a mars-class ship, a minimum safety factor of 3 (really, six) for fuel, water, air, food, medical supplies etc), instead of a 3 year mission with only a safety factor of 2.
#Apollo 10 was functionally identical to Apollo 11, to the extent that the only difference was that they did not provide enough fuel on the lander/ascent module to return to lunar orbit, to deter Apollo 10 astronauts from "going rogue" and attempting a landing ahead of schedule
Also, for completeness sake, Apollo 1 crew died during testing in a fire; Gus Grissom, Edward H. White II, Roger B. Chaffee
Doing all of that with no actual plans to land seems silly.
Actually landing people on Venus could only be a disastrous mistake.
Simulate what ? Sitting in place ? We can do that on earth orbit. Engines running for long time ? No need for people aboard for that.
Not with chemical rockets. Perhaps you meant "engines sitting idle for years in interplanetary space, and then restarting reliably".
Nitpick: Apollo 11, 12 and 14-17 were boots on the ground. 13 had no boots on the ground.