I think injury risk can be quite high if you overdo it but as long as you don't train it too often (like 5+ days a week), make sure you're a bit warmed up, and don't max every time, I believe the risk is not that much worse than other types of strenght training. I guess the risk is slightly higher overall even if you train smart just because it's smaller muscles involved.
To answer your other question, there are definitely more ways of getting a stronger grip. Holding your own bodyweight for X amount of time is one of them. Some other things you can do that I occasionally do myself:
- Not use grip straps often (unless you're going super heavy I guess. I personally never use straps).
- Doing farmers walk, or simply hold heavy dumbbells or a barbell while standing still.
- Wrist curls.
- Do barbell curls with palms down (I like this exercise, have your thumb on the same side as your fingers to make it heavier).
- Plate pinch (I think this exercise can improve your grip quite a bit but I've only done it like twice, it's popular among people with insane grip).
- Some people recommend
Expand-Your-Hand-Bands to complement the Captains of Crush.
Those are just a few popular exercises. Using grippers is my personal favourite way of training grip.
It's good to probably have at least three CoC grippers: one for warmup, one for working sets, and one you can close like only one or two times, or maybe not at all. If you're new to strenght training and grip ("normal" male strength), I'd recommend buying the Sport, the Trainer, and the #1. Most guys who lift weights every now and then can't close the #1 the first time they try.
If you are stronger than average or have done some strenght training, then maybe the Trainer, #1 and 1.5# is a good start.
It's probably smart to mostly train with a full range of motion too, just like other exercises. The reps I do in the video are called MMS reps and are not full. Warmup sets are usually 15-25 reps or so, working sets 5-10, and really heavy 1-3. Some people also like to take a gripper they can't really close and hold it as closed as possible for as long as possible, I don't like to do this and don't recommend it to someone just starting out.
I hope some of that helps, I could write more later but gotta go to bed now.