Don't I know that job from a previous boat, so I hear you loud and clear!
Luckily Stella is all freshwater flushing. I had occasion to be servicing the heads (joker valves and macerators) earlier this year and I had a look at the hoses. There was some build up but nothing like I have seen with salt-water systems. I gave them a ream-out regardless with some rigging wire, modified at the end to look like a "brush". Also, they are still flexible along their length.
I was really happy with the easy access to tanks and pipes that the Amel design affords. On other boats I've had to cut through fibreglass to get at things!
Good to know that the phosphoric acid products work on those lumpy bits.