Few additional thoughts / pointers for you in no particular order (I have similar shrimp tanks with Amazonia sub and have been doing lots of testing and measuring lately with SS GH+ minerals):
Firstly, calibrate your TDS meter. It might be new, but chances are that it is still "off". Get some TDS calibration "standard solution", cheap enough off Amazon etc, and calibrate. 342ppm NaCL solution is pretty standard. Before then, don't try and compare your TDS with anything (and going forward, take anyone else's TDS measurements with a large grain of remineralising salts unless you believe their meter is calibrated too!).
Having said that, have a read of my previous post in this thread which lists the TDS (ppm) readings that you should expect with Salty Shrimp GH+ at 5, 6 or 7 GH. Mt meter is a decent HM Digital model and calibrated last week, so I am as confident as I will ever be about ppm's, but still please don't take them as exact. I go with 7GH like what @Zoidburg
said above for my RCS:
Remineralising to 7 dGH using 1.75g of SS GH+ per 10 litres of RO water gives a TDS of about 150 ppm.
The old water in your tank will be at a much higher TDS than your water change water because of all the other stuff you add to your tank (e.g. fert's, food, etc). My RCS tank is currently sat at about 300ppm (despite w/c's with 150ppm water as above) but the shrimp seem happy with that. When @Zoidburg
said "TDS should be between 125-150" I think he means your fresh, newly remineralised RO water, not the tank itself.
Stability is arguably more important than any specific water parameter, so don't try to change anything suddenly to match what someone else says your tank should be at. RCS are pretty adaptable so don't fix it if it aint broken. 20% water changes *could* be on the high side depending upon how much it causes the tank water parameters to fluctuate. I've been aiming for 10% changes, maybe just 5% if nothing is too high.
The TDS of your tank water is GH + KH + other stuff. If your tank TDS is high, you need to know whether the tank water is too hard (GH and/or KH high) or just "dirty" (i.e. correct GH / KH but too much other stuff). My point being that you could do water changes with pure RO to bring the TDS down, but a TDS reading doesn't tell you what the "staff" in the water is, and you could theoretically end up with a tank with a wonderful TDS of 150-200, but with 0 GH and 0KH (i.e. just "other stuff").
One degree of hardness = 18ppm roughly. So if your water is 7 GH and 2 KH then that's (7 + 2) x 18 = 162ppm. If you're TDS pen reads 200 ppm then you know that there is 38ppm of "other stuff". TDS readings by themselves are not helpful unless you know how much of ppm is made up of necessary minerals (GH and / or KH) and hence how much is the "other stuff" (fert's, food, dechlorinator, etc, etc).
Assuming you're using the API pH test kit, then pH 6.0 is the lowest reading on the scale, but the actual pH could be 4 or 5, you have no way of knowing. Not a problem, but just helpful to realise that the test result is quite possibly less than 6! Also, if your water is 0 KH then it seems that most pH measurements could well be off anyway.
If you want to get more accuracy from API GH / KH test kits, use twice the amount of water then each drop equates to 0.5 degrees hardness (rather that 1). Sometimes useful when you are initially working out how much you need to add to your w/c bucket.
I know the pot says that the SS minerals dissolve "almost immediately", but I have found that you actually need to give them time to mix and dissolve properly before you take GH or TDS readings. If you do this immediately, the water may read about 1 dGH less than it actually becomes an hour or so later. Best to mix up water day before water change if possible and leave with a powerhead / pump / airstone running to ensure minerals properly and fully dissolved. But with the SS, I've found that about 1/2 hour is usually enough for TDS to settle down.
Hope there's some useful pointers here. Happy shrimping!