I watchBattery health at 85% but loses charge in less than 10 hours. Have unpaired and rebooted

Because you have done the main troubleshooting you can move onto looking at the iphone software.

First of all make sure the iphone is not running VPN, Adblockers, (check both in settings, General, VPN & device mgmt) or any security apps

Then look at Apple Watch battery drain - the ultimate … - Apple Community

Batteries - ALL BATTERIES over time lose their ability to hold the same charge they did when new. There is no way to bring a battery whose battery health has declined to 85% back to new, except to replace the battery.

I agree with lobsterghost1 you cannot get it to the original nominal 18 hours without a new battery.

However it should still last 85% of the nominal 18 hours for the same workload. So try the software trail in my link.