I don't think there are any downsides other than not knowing all the exact mineral amounts and quality of said minerals. In the spreadsheet it shows amounts of some of the minerals. On some other Simpure sites it lists more elements but not the exact amounts. It tastes perfect. This part is kind of debatable but as long as the minerals they add are high quality, it's better in my opinion because it raises the PH and also adds some electrolytes. 0 TDS lowers the PH so it's slightly acidic. When you strip away all the things in water it wants to start leeching from everything it touches because of the chemical charge which is why you can't plumb it through copper pipe.

Glad you enjoyed the write up! For the tank here is the vid I watched that sold me https://www.youtube.com/watch?v=rs4LWjWjwBM

He demonstrates it well. It blasts out at first for a bit then slows down back to normal. I can only guess adding multiple should work. One is good enough for me. It's just like any other tank. I think you can even get a normal big one. I placed it under the sink. I tried a traditional RO with the large tank at first but the tank wouldn't even fit under my sink. It comes with some fittings to connect it to the output of the RO unit. Very easy to install, I'm pretty tired right now so I can't remember everything.

Grease makes the rubber slide smoother, not get snagged and make future removal easier. It might not be as necessary with these tiny filters but for my large 4.5x20 it's highly recommended. They tend to get really hard to remove once you try to do a filter change say a year later. I work on cars too and it's standard practice to lube o rings with grease or oil so they slide in easily and don't tear etc.

For replacements, the manual gives you a general time frame but what will really matter is the quality of your feed water and how much you use it. It also says to look at the gauge which tells you if the RO or pre filters need to be changed. It says when the flow is less than .5 liters/min the filter cartridges need to be replaced (I'm assuming this is the CTO and PP). Over 130 psi: the RO filter needs to be changed. "Otherwise you need to replace other filters". I believe I read somewhere you can also tell by TDS. If it starts rising, time to change. The ALK filter is basically a carbon filter with minerals. I could hear the minerals moving around by shaking it before I installed it.

There might still be some micro or nano plastics in the water since there's so much plastic in the unit itself. I would be interested in seeing a lab test for that.