I'm sorry, how is sending humans to Venus, already a long flight, to experience conditions significantly different than those on Mars, going to help us more than say, sending a crew up to ISS for eight months, then transferring them to a craft to send them to the moon, to have them live on the moon for a few days or weeks, to then send them back to the station for eight months, to then send them back to earth?
We could send a rescue mission straight from Earth to the moon a hell of a lot faster than we could send one to Venus, and we already have experience with landing on and taking off from the moon, so developing craft to do that mission would be much easier, and could be part of a greater set of missions to the moon in general.
If we really want to test sending humans outside of the region of space protected by Earth's magnetic field, how about sending astronauts in a solar orbit, to end up at the moon? If we have existing lunar missions, we could even have a short-flight team waiting for them when they arrive, to study their physiology from the long duration flight and recovery in a lower-gravity environment, without those doing the study necessarily being subject to it themselves.