Number one suggestion is a complete ban on anyone putting paper down the heads.
The hoses suffer a build up of deposits due to reaction between seawater and urine. It is unavoidable unless you change to freshwater flushing. Hoses should always be installed so that it is possible to remove and replace them. I don't know the layout on your boat but suggest you investigate further. If you can get the hose out either replace it or give it a good bashing on something hard to break up the deposits so that they can be removed.