Over-training is completely relative. No one but you knows how intense your clamping is, and every body is different (not to mention sleep and nutrition). You just need to learn yourself and pay attention to the signs.
If you have no trouble outside the clamp, it’s not likely an over-training issue. Personally, I always have to clamp down most of the way while soft because the clamp will never go tight enough when I’m already hard. Maybe your issue is you think it’s tight enough because you are too hard before the clamp goes on. Also, make sure you are stimulating the whole time. Clamping makes the tip more sensitive, but it doesn’t stroke itself…
Try this way: lube up a little (coconut oil is perfect), close the clamp a couple clicks, and pull it as close to your body/balls as possible with one hand while stretching mr member with the other hand. While it’s held tight in, click down as much as you reasonably can in that position, and hold it there with free hand while edging up your hard on with the stroke hand. When it gets fully stimulated it is properly “ballooned”, and the effect will be that it is not as /stiff/ as a natural hardon, but the skin and veins will be tight and poppin’. The clamp must be sufficiently smaller than your erection is normally, creating that pinched-off “balloon” state.