Tendonitis is a real concern, and you surely have the right avoidance program. Regarding the CoC grippers, when I do up to 5 sets of 5 reps twice a week, there is no tendonitis. For me, raising the sets or the reps or the frequency above those numbers results in tendonitis.
Keeping grip work to twice a week sessions of 5 of 5 means I can move to the next stronger gripper after about 30 exercise sessions (15 weeks or so).