Yes, running separated resin. Anion is depleting first. I'm getting around 50 gallons this way. TDS from the tap is usually 500+. Not sure if the water softener is affecting this number or not . TDS before the resin is around 15 Iif I remember right.
The anion resin depleting first is to be expected. The ions that the cation resin exchanges makes this happen first. I run separate beds and my anion depletes quickly, so much so that I added a second anion stage. I now run cation > anion > anion > mixed bed with color changing cation.
If you were getting 5 ppm after your DI resin, you'll want to replace the last resin in the series, the pro mixed bed. The reason for this is because the color change is on the cation resin, which get depleted first if your system is functioning properly. However, if you let your anion resin deplete, the uncolored beads in the final stage (the anion beads) are probably depleted. You just don't have a way to see it.
The water softener is not an issue. If anything, it likely makes it easier for your RO unit to function, as softened water is easier to filter than unsoftened high-TDS tap water.
You're going to have a tough time filtering your water because of how dirty your source is. You can make significant progress if you maintain your RO system well though. First, change the prefilters regularly. Either change them on a 6 month schedule or test them to see when they need to be changed. The sediment filter needs to be changed when the pressure in the system drops significantly. Put a pressure gauge before and after the prefilters to measure this. Once the pressure drops, it's time to change the sediment filter. The carbon block(s) need to be replaced when you detect chlorine/chloramines in the waste water from your RO membrane. To do this, buy some simple test strips and test the waste water every so often. If you see chlorine or chloramines, you need to replace your carbon blocks.
For prefilters, always use a very small sediment filter (less than 1 micron if you can find it, 0.6 micron or smaller would be ideal). For carbon blocks, any size will work so long as it's larger than the sediment filter size.
For your RO membrane, you don't need to do much except give it good pressure. If you don't have pressure above 50 PSI, a booster pump is probably a good idea because of your source water. Also, make sure you have a 4 to 1 waste to product ratio. Some people can get away with running less (3:1, 2:1, etc), but you absolutely can't. Because of your dirty source water, you WILL ruin your membrane early if you starve it of the waste water it needs. It sounds like your 50GPD membrane is performing fine, but consider getting a 75GPD membrane when it needs to be replaced. The 75 GPD membranes typically have better rejection rates.
A DI resin setup like the one you have is likely to be the most cost effective, but you really have to keep up on the resin changes. Consider adding a second anion stage if it depletes that quickly. Also something you should really consider is a degassing chamber for your RO water. CO2 could be the reason your DI resin is depleting so quickly, and aerating the water before you run it through your DI could dramatically improve your DI lifespan.
