I like your ideas, but unfortunately hard to get into that sort of stuff without at least a building/construction background.
And plumbing is tricky in that there are usually lots of rules, some to protect the user, some to protect the system (eg so bad water can't go back up into the system). I think flushing toilets with drinking water is crazy, it should be grey water, maybe a mix of rain water and maybe shower water. Grey water is interesting in that it could make a big difference, but unless its planned for before building its tricky to add later as much of the plumbing is under slabs.
I think it is good to try and lever some of your current experience, that could get you into a new field quickly.
If you can afford to retrain then go for it.
But unfortunately money does tend to be a big factor in any of these decisions!