Just like the moon, we did not attempt to land there on the first go-around. Neil Armstrong was on Apollo 11. Apollo 4-6 were unmanned launches, 7-9 were testing the hardware with humans, mostly with "free return" orbits where even if the engines didn't start again, they would land back on earth in about a week. Apollo 10 was a wet rehersal that did everything except land on the moon#, including "parking" in lunar orbit, then starting up the engines again and going back to earth; finally Apollo 11-17 actually were "boots on the ground"

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