Over 120 is way over. You need a pressure regulator for your entire house if that reading is correct. Your plumbing and sink fawcet valves may fail you at too high a pressure. Good that you cut it back for your RO. The higher pressure, to a point at least, will give you better output but they do have a rating. The manufacturers make pressure pumps to boost pressure to get more production, but, over 120 seems excessive to me. You have to check the rating for your membrane and see what it is. Makes sense the TDS would drop if you lower pressure but a half open valve is not a perfect solution. Get a regulator if you want it right.

Joshua, all membranes are not created equal. Some allow nearly 5% of the solids to pass through, others allow kess than 2% at their specified temperature and pressure. Different companies may use the same brands, but, there are different performance specs for different rated membranes. Thats coming from Kent lit, so if others buy from one source, maybe they all buy only one grade? Theres a lot of sales hype over all this too and that throws off the real facts of the matter.
Since your prefilters don't keep size, thats probably not what I am interested in. Mine stay what they are always, wet or dry.