Your comment about "chewing on the leather belt" caught my interest. So I just did a quick "google" and came up with a web site that will give you an idea on the chemicals used in the different tanning methods. Thought I'd enclose it for your interest.
If there is a pet toy made with "pet friendly leather'", and you can trust the manurfacturer, then that is possibly the way to go.
In regards to the wire chewing, I can sympathize with you - it gets extremely expensive. We've lost 4 mice, - not the 4 legged type - yards of stereo cable wire - our surround sound stereo has become mono - The cable technicians came and put brand new coax in the house and three days later we had no TV - chewed the coax cable. A couple of headsets and microphones, not to mention small power supplies. Plus a myriad of boxes, plastic, containers, and all from mainly one cat. What I have noticed though, is any of the electrical cords carrying line current - 110 120 volt - seem to be to the greatest extent, off limits, for some unknown reason. The cords may receive a little bite, but not enough to penetrate through the insulation. But if they are unplugged, they're fair game and the cords could end up severed.
We've had this condition since BB was a kitten. At the time, I was talking to one of our friends, who is a microbiologist, and she suggested sending a sample of fur so she could do a chamical analysis. This was because as a kitten she had a very tough start in life. We thought perhaps there could be deficiencies in her system. One of the major deficiencies that our friend found was Zinc. We gave it to her for a while. Then there were a couple of household moves and we forgot about this supplementation. We are back into it again. Since the end of July/first week in August, I put a question on this site re this situation - regarding cat chewing wire - and someone suggested that we try something called "Bitter Apple" that you can purchase at the pet food store. I took it and manually rubbed it into all the wires by hand, and that seemed to help somewhat. A short time after, I remembered the Zinc deficiency and started to supplement it again and basically since the first of September, all chewing of anything other than food has ceased.
A few days ago she was looking at the telephone wire and she just passed by it.
Maybe you could be having the same sort of problem - some sort of defieiency in your animals. And they're looking for a "fix".
Anyway, try the Bitter Apple. Hope my story might give you some ideas.