You don't need or want a softener AND an RO unit. The softeners generally use some type of ion exchange and leave you with something else undesirable in the water (sodium usually). The RO unit will remove everything from the water and is all you need, then remineralize. HTH.
The softner isn't exactly for the tank. That one is more for me, cleaning and hygiene purposes. I also hear it makes it easier on the RO unit, but that could just be a myth online I've read. The RO in my perspective after the softner is to lower the TDS and get rid of the salts the softner will add. Also, I don't actually want to drink water from a softner.
It sounds like you have done your research, and using RO is an effective way to start with 'nothing in the water', and then build it back up to your liking.
The equilibrium will restore the major salts, and the trace products will restore the trace elements required for fish (Fresh trace) and plants (Seachem Trace). There are some small traces of copper in the trace products, so some research will need to be done to ensure you keep the Cu ppm below what is dangerous for shrimp (I don't know what that is).
You mentioned underdosing Flourish (comprehensive). I'm not sure why that would be necessary, as it contains necessary micro elements as well, and should not overlap much with the products mentioned. Maybe someone has a better opinion on Flourish.
What is missing is an alkaline buffer. You mentioned correctly that your pH may go too low. You can add enough of the alkaline buffer to stop that from happening. Since you like Seachem, look into Seachem Alkaline Buffer. Good luck creating a nice South American environment.
The only reason I mentioned under-dosing, is because I prefer a slower growth. Since I have no CO2, the lighting can't be high, and it's kind of an approach for me to keep the ferts slightly lower than the directions to keep a balance. I've had success so far, but probably when my TDS goes down, I might have to revisit that.
I've bought every API liquid test that I could find, so that included the copper test. I assume as long as it stays undetectable to API's test, it'll be fine for the future shrimp? It seems copper levels for shrimp havn't been zeroed in yet and is pretty debated. I guess if it becomes to much of a hassle, I might give up shrimp.
Does anyone know if I use an Alkaline Buffer, if it'll be okay to recycle old water to use as indoor plant food/water?