Hey Eliott
I used to wonder the same thing if something was organic and natural why would I need to flush it? So one time i didn't, and the buds ended up with the most odd taste. I think if you're feeding low levels then you don't need to do a two week flush but I would recommend doing at least 2 waterings of plain water prior to harvest.
You can do partial harvest but be mindful of contaminants getting into the cut sites. I would do your last week and water plain ph'd water and then evaluate whether it needs to be harvested in sections or if it can wait an additional day or two and harvest all together.
Happy harvesting. ✌️