The tiny thought I had forming yesterday was about which part of your RO/DI was failing. The membrane is still doing a lot so it came down to the resin. I'm actually glad to hear the filters haven't been changed. Saves a bunch of speculation on my part and questions to you. The sediment filter is dirt cheap, and the carbon block isn't much more. They need replacing. Although they do a lot of filtering themselves, they also do protect the RO membrane, equally important. Get a new DI cartridge too, they really should never be used to the point of full exhaustion (of either the anion OR cation resin), weird/bad ion exchange can start happening.
The membrane may still be fine, I would do the other stuff first and see what happens UNLESS you do not have a flush kit on it. Being able to flush the membrane makes a big difference IMO. If it's never been flushed it is covered in crud (scientific term). If you have money to burn, I'd just get a new one for the heck of it. You don't have to use it immediately and it will need to be replaced sooner or later.
I used to try to test the pH of RO/DI water until I figured out it was pretty pointless, too pure to get a real reading, drops or meter. But for the record, I consistently came up with a pH of 5.5 or so.
The pre-filters and RO membrane are pretty straightforward. Only stuff the about the size of, or smaller than water molecules can get through the membrane. The DI resin can get most of that.
Here's what took me a long time to *get*. DI resin doesn't absorb anything. It ONLY exchanges ions. Positive ions, called cations, and are EXCHANGED for hydrogen ions. Negatively charged ions, called anions are EXCHANGED for hydroxide ions. Maybe you knew this, I had heard the words but didn't fully comprehend it until I started recharging DI resin. If something has made it through the RO membrane but but does not have an ionic charge it's also going to go through the DI resin.
Short diversion: The reason you never want your anion or cation resin to get fully exhausted is because some ions have a stronger charger, are *stickier*, than others. Once the hydrogen or hydroxide ions are gone, you have no idea what kind of ions are replacing others. You can have a not-bad/very-sticky ion replace a very-bad/not-sticky ion which is then passed out of the resin. Once the cation OR anion resin is exhausted, you can actually be getting out worse water than goes in. This too I learned the hard way.
A TDS meter really only reads conductivity. Not everything is conductive. So even after your RO membrane and the DI resin, there can still be stuff in your water which you can't measure. Not much... but it isn't the same as distilled. You could put a little sugar in distilled water but it's not going to show up on a TDS meter, it's not conductive. So that Total Dissolved Solids isn't really all that *total*. Other more obvious things it won't detect is stuff like CO2. Then there is more confusing stuff like how not all ions are equally charged/conductive. And the water temperature affects readings too... If you get a reading of 200 ppm in very cold water then heat that water, the reading can go up. Some/most meters do have electronic compensation but it's not perfect.
Sorry I had to use so many words, but what I am try to say is even with all brand new filters, there can still be a tiny bit of stuff other than pure water after the RO/DI process, even if you have a 0 ppm reading on your TDS meter. With your old filters, especially the DI resin, who knows what is actually in that final RO/DI water.
I am still not clear how you are arriving at a consistent mix of tap and RO/DI if you are not mixing it before putting it in the tank. Have you considered a barrel? Fill'er up with whatever ratio you want, add Prime or something, drop in a water pump to circulate it and a heater to get it to the right temp, next day it's ready to go and very stable. Any tests or readings you do on it are going to be more accurate. If it's anywhere near the tanks you can put a piece of tubing on that pump you have been using to circulate the water and just pump it out to the tank(s). Try it, you'll love it!
If you cannot get the combo of pH, kH, gH and TDS which you really want/need just by changing the ration of tap and RO/DI water, you can adjust it in the barrel. Although my philosophy is to add as few potions and powders to the water as possible (excluding ferts) sometimes you may have to. Brewing it up in a barrel is a whole lot safer.
I think there is a bandaid solution which you could use immediately but there is no way you could do it unless you were premixing the water in a barrel. It's kind of unconventional (understatement) but I have tried it and it has worked, even for delicate fish and new fry. Don't really want to post it here because if done wrong it can go very badly.