Sup brozif, I like the new set up, definitely seeing your ideas come together. Just some thoughts on the things you said and the things you are doing, you absolutely can get through an entire grow without doing a res change, salt build up can be a problem, but their are products that can help to prevent that from happening, not to mention just keeping the ppm's low can prevent it. But I do recommend changing between the veg and flower stage, just to get all that veg juice out of there and jump start them straight into flowering. And that's always a prime op to scrub your res if you have the time, or just swap out with a spare you have ready. I know the idea is to save as much time as possible and do as little work as possible and still get a good product, so I think those are some ideas that will help out without costing too much time. Because the biggest bummer in the world for someone who does this with little time is for the crop to get sick or to get a tox. When your time is valuable, any of it being wasted is a massive heart crushing bummer.
Duct tape is fine, our friend must have been misinformed, but that's okay, that's why this forum exists.
Remember that even seedlings can rock out with 250ppm, so if you need to go to that point, or even up to 280 you will be fine, I think you are at a .7tds factor, right? Don't let other fall behind to let some catch up, the first two weeks of the plants life are among the very most important, and a good start is a expedited launch to a good finish. So if the others don't catch up, cull them, I know with DWC it'll be only a short manner of time before your 3 survivors fill up the entire space.
Oh, and as always, take your own path, man. Win or lose, the lessons you learn will sculpt you into a masterful grower soon enough. Do whatever you need to do to get to a complete crop, do not be weary of breaking away from the game plan if it's called for. I think with regular maintenance you won't come into any problems besides a possible salt build up. That's my primary concern, not that the plants can't handle a lot of salts, because they can, but it can affect how it burns.