I always tell RO/DI users to use the smallest micron range sediment filter they can and make sure it is absolute rated versus the less efficient nominal rated.
In my personal system I use a 0.2 micron pleated absolute rated sediment filter, a single 0.5 micron near absolute carbon block them my RO membrane followed by dual DI.
In almost 20 years of RO/DI ownership I have never found need for two carbon blocks and most vendors who still suply them that way have not done their research. Carbon technology and carbon block extrusion and manufactiring has come leaps and bounds in the last decade or two. The old method of dual carbons served two purposes, 1. the first carbon was used as a sacrificial filter to make up for the high micron sediment filter many insist on using and trapped all the things the less expensive sediment filter should be trapping so it is useless at adsorbing chlorine after a short period since all its pores are fouled and 2. Older carbon did not have the adsorptive capacity of modern carbons and many lasted as little as 300 total gallons( 60 gallons treated and 240 gallons waste at the normal 4:1 waste ratio) so they installed two to give you any chlorine removal capacity at all. Even today there is a vast difference in carbon blocks and their lifespans, a 10 micron carbon or GAC can still last as little as 300 to 1,500 total gallons (treated +waste), a 5 micron as little as 3,000 to 6,000 total gallons, 1 micron 9,000 to 12,000 total and 0.6 and 0.5 micron as much as 20,000 gallons with a single carbon block.

Every filter and canister you place in front of the membrane has an associated headloss which reduces the rejection rate or removal efficiency of the RO membrane making the DI work harder to make up for what the membrane misses and driving your cost of operation up.

I have a pressure gauge on the incoming tap water and after the carbon block so I can monitor headloss at a glance and helps me trubleshoot the system more effcetively. I also use a low range chlorine test kit to monitor the effectiveness of my carbon block at removing chlorine.

What are you using to test the phosphates in the finished water? I spent some time with an organic chemist a few days ago and heard some doubts expressed about the Hanna Checker instrument. For one, it is extremely hard to test ultrapure waters with any degree of accuracy and repeatability. Think about it, we are talking about parts per million or parts per billion here. To put that in perspective 1 ppm is like 1 penny in $10,000, 1 minute in 1.9 years, 1 inch in 15.8 miles or 1 ounce in approximately 32 tons. It is miniscule! A ppb is like 3 seconds out of an entire century. We are asking a small handheld hobbyist instrument to measure something that small and we are not in a controlled environemnt like a clean room or lab using sterilized instrumentation and apparatus and expecting accuracy down to these levels? I don't think so!

Even one hard water deposit or grain of dust in the glass you use to test in or your test tube will skew the results dramatically. Take the same test 10 times in a row and the results can be all over the place unless you do things in exactly the same way every time, use the same glass washed and rinsed the same every time, draw the same exact amount of sample each time with a titrator or let it mix or sit for exactlty the same amount of seconds etc. Hobbyist grade instruments can get you in trouble when you are depending on lab grade results and personally I don't lend much credence to them. I look at them more like a presence absence or litmus paper test, you have phosphates at a measurable level or you don't.

If you do suspect phosphates in your RO/DI water I would look at two things first, improving the rejection rate or removal efficiency of your RO membrane since it removes 90-98% of the contaminants and improving your DI resin and/or bed contact time in the DI canister(s) to get maximum treatment. The sediment can carbon filters have little effect if any of contaminants such as phosphates or TDS in general but they are important for protecting the RO membrane and ultimately the DI filter. Another thing you can do is to install a water softener if you do not already use softened water as this takes a huge load off the RO membrane.