Sure, I could go on about more water treatment.
A) If you are dealing with something like hard water (high GH) and you want to try peat moss here is the method that I found worked best:
1) Remove most of the minerals first with a method like RO. It does not have to be perfect, and it is just fine to make a blend of RO and tap water (as long as the tap water is acceptable) Or use pure RO and remineralize.
2) Use peat moss more for the tannins and black water effect, not so much for the softening effect, though it will remove some minerals. The effect varies. I have seen some hobbyist posts about peat moss that lowered the GH, KH and pH significantly. I have seen only minor changes. I think it is possible that I am not using much peat moss in what I do, but it may also be the different peat mosses do different amounts of softening.
If used in tank via the filtering method each water change raises tank levels immediately back to source levels in a rapid increase and then the levels tappers back off, ....
This is the other part that I usually go into in more detail when discussing water treatment. If you want to go this route (peat moss or any other water treatment) for your tank then you need to prepare the water ahead of time for water changes so that the new water matches the tank water. Do not depend on in-filter treatments to handle the water that you add for a water change. The varying parameters are not good for the fish and the volume in the filter is small, and anything that will work will get used up quickly.
Run a blend of RO + tap into a garbage can.
Turn on a pump that circulates the water.
Aim the pump to flow through a nylon stocking of peat moss. (Knee-hi, filled with peat for a garbage can from 20 to 44 gallons)
B) I agree that the water softening resin pouches are not really much good, and indeed they are rejuvenated by soaking in salt. Just like the sodium exchange water softeners that are so common in many homes. It is the same concept. When I tried them in a small tank I could see the difference in the GH test, but it did not last, of course. The next water change raised the level right back up, and the resin pouch was exhausted, required rejuvenation. I threw it out.
C) To properly alter the water to suit the fish you do indeed need to look at all the parameters, including the TDS. Exchanging one ion for another is not lowering the TDS. If your livestock needs the lower TDS then that is the first step. Remove a lot of the 'stuff' from your water. It is easier to remove pretty much everything then remineralize with the right things rather than trying to customize the removals, unless the simple filtering procedures available at the hobby levels suits your tap water.
d) RO membranes remove sodium easier than they remove calcium. Calcium can foul the membrane through scaling, and you have to replace the membrane more often. Doing a pretreatment of running the water through a sodium exchange water softener before running it though the RO will increase the life of the RO membrane.
Interesting that you got 2 such totally different water parameters from 2 different RO units. Was the membrane wearing out?
My water started off better than yours, softer and no iron, but the water company added sodium hydroxide to raise the pH, and chloramine.
The RO unit I had was a 2-part system that softened the water a lot for the whole house, then sent some through the RO in the kitchen. It included activated carbon filtration, particle filter, and sodium exchange as the treatment before the RO unit. The first treatment was really good, produced significantly softer (lower GH) water, and I had several species of fish breeding in it. The RO + just a bit of that water was for even softer water fish. Rams bred in that.
The first clue that my RO unit was getting old was rising GH in the water straight from the RO unit. Since I was adding some tap water anyway for the minerals, I just used less tap water. However the whole system died about then, it was about 20 years old, so I never did figure out which part needed service.
We had originally purchased the whole house + RO unit because we could smell the chlorine (not chloramine) used back then in water treatment. The smell was gone in the treated water. I was not keeping fish that long ago, to test the water.