Here is my take based on what you have, and if you purchase the Puratek Deluxe from marine depot.
Use the Puratek as a 1 micron sediment, followed by 2 carbon blocks, lead that into a RO, followed by another RO (in series) take that out to your current 3 DI cartridges (not sure how your running these, all mixed?, cation, anion, mixed?)
Since you will have extra housings from your old build, you could even run another sediment like a 5 micron, but if you do this I would go with a reusable 5 micron like this
one. It's pricey but you shouldn't really ever have a need to replace it, just periodically back flush it and clean it up. This way large particulates can be caught in the 5 micron, and it will save your 1 micron.
Another thing you could do is use a 4 stage DI since again you have the cartridges, if you were doing this I'd say run a cation, followed by your anion, then 2 stages of mixed bed. If you monitor each stage and replace as needed, the 4th stage (mixed bed) would hardly need replacing and would act as a super clean polishing filter since it would hardly be pulling any TDS out of the water by this stage. Or if you know you run through cation cartridges quicker, you could run 2 cation cartridges, followed by anion, and then a mixed bed, if anion depletes quicker you could do it the other way with cation, anion, anion, mixed bed.
