The best choice for more agility is a bike with a front wheel attached to a frame. They are the best, because they provide a more complete mobility, and you need not worry about a wheel or a pedal. A lot of people think that bikes with front wheels will get the worst possible posture, because it looks weird, and because they have to move a lot. But it's true that it will be uncomfortable. But there are some advantages too. You can't lift your legs too high, so they are not "overly" low. And you don't have to worry about bending your body. Your legs are in front, so they're a bit stiff. This means that you can ride a lot more, but your body is not going to get too "stretched" and the posture will not be bad. In fact, I think you can have a pretty good balance between comfort and movement.
The problem is that the problem is not always obvious. So we need to make sure that we don't fall into the trap of thinking that the problem exists only for people with bad posture, or that only those who have bad posture have the problem.