Using DI only is a bad idea. I wouldn't even consider it. You need a carbon block for chlorine or chloramine removal. So the first step is to see which your city water uses.
It also seems a few things were left out about when you change your filters and the components that help you to determine that (not a tds pen). You don't just change the filters at a set time period. Some people will need to change them faster, others slower.
The first filter, the sediment, will start to turn brown. You should have a pressure gauge inline before the membrane (the 3rd filter) that tells you the psi (pressure) going into the membrane. Take note of your pressure with all brand new filters. When the pressure starts to drop, it means the sediment filter is starting to clog. That is a good time to change it. There are different density sediment filters. With 89 tds from the tap, a 1 micron would be good. Knowing your pressure also lets you know if you need a booster pump or not.
Then your chlorine/chloramine filter, the carbon block. You need a free and total chlorine strip to dip in the water exiting the unit. When you notice ANY reading, you change it. Or you can change it when you change the sediment filter to play it safe. A 1 micron carbon block woild also be ideal.
The membrane is the heart of the unit and doesn't get regular replacement. I've had membranes last 3+ years with higher tds than 89. It goes by the rejection rate. Once it stops removing roighly 90% of the tds, you change that.
The DI is usually color changing and will let you know.
But make sure you have an inline pressure gauge. It's important to know the pressure for multiple reasons. Also an inline tds monitor to tell you the tds before and after the membrane. That's how you know rejection rate. There are many affordable units that come with these things included. If they're not included, you'll need to add your own.
