I think you're overthinking it. I check my cage every day. If something looks wrong I fix it.
If toys are worn out I replace them. As far as pumice I've never washed that before but it is something to consider. I'd say smell it and if it smells bad it's time to do something about it. I try to change out the wood shelving and flooring and deep clean the cage about once a year. One time I had a chin come down with coccidiosis and did it right before he came off the antibiotics. Chinchillas are very easy to take care of.