A 30-frame animation at 30fps is only going to be one second in length. I'd lengthen it to at least two or three seconds, personally.

Yes, spline interpolation should allow for smooth motion between keyframes.

Bear in mind that, depending upon your system and the complexity of the scene, Poser might not be able to play back every frame of the animation in the real-time preview. By default, Poser skips frames if it can't keep up, which can make the motion seem jerky.

Disable "Skip Frames" from the animation controls or the Animation menu to force Poser to play them all. Box tracking can speed things up, if necessary.