Unless you’re seeing issues with devices dropping off or Zigbee commands not getting through, I wouldn’t worry too much about it.
That warning is simply a warning - if you’re doing your best to follow the best practices with regards to zigbee, then you can safely ignore it as long as everything’s working fine.
In your case, you seem to be doing whatever you can, and with 100 devices, 85% isn’t bad unless the ZHA diagnostics reports an actual usage which is much higher than that.