There is not much wisdom to share. As you realize, a water change using source water with 40ppm nitrates will raise your tank nitrates.
NitratEx appears to be a similar resin as API's Nitra-Zorb.
You could use your water and just swap out NitratEx pouches from your filter as needed to keep nitrates at bay. Be advised that like Nitra-Zorb, the use life of the product is likely compromised as detritus coats the surface of the resin pellets. Best to place it in the filter after very fine media like polyester fiber,
You might also use this product to pre-filter water for water changes. Before I came up with my DIY filter, I setup an unused 10g aquarium and used Nitra-Zorb pouches in the filter to filer fresh, clean water. I'd repeat the process a few times to get the water volume I needed for water changes, recharging the Nitra-Zorb as needed. You might use an inexpensive Sterilite or Rubbermade bin instead of an aquarium.
Alternatively there is the Pozzani filter as well as other inline nitrate filters on the market. I can't speak to how practical they are as I'm uncertain if they can be recharged. If cartridges need to be replaced, depending on use life, they can sometimes be more expensive than simply buying nitrate free spring water.
You could also invest in an RO unit, however good units can be pricey and then there's the cost and hassle of replenishing necessary minerals.
Last, perhaps you have a family member or friend nearby that has nitrate free water that you can 'borrow' for your weekly water changes. I don't know the size of your tank, and hauling 5g buckets of water isn't fun, but it is an option.
Best wishes from another hobbyist in the 'Nitrate Fight'.