Don't have advice on your plumbing question--suggest you look at info from lectra-san on that one.
I don't see your home port so don't know where you're cruising from. Here in the USA recently there has been guidance from USCG that lectra-san requires a holding tank (as you seem to know) use in no-discharge zone. What you may not know is that, if you are a US flagged vessel, a direct discharge isn't authorized to exist in any recreational vessel unless it was installed prior to 1979. I don't think they had lectra-san systems before then, so if you are a US boat, you might want to keep the holding tank in place or verify with the USCG (or a US marine surveyor) what is legal here for new installs.
Best of luck to you,