Yeah, ideally an RO membrane would be in the way, and the sedimentary pre-filter and carbon block precedes that, but as discussed, RO is too slow and requires a tank. A carbon block and pre-filter are both very cheap and with nothing preceding the resin, it can clog rather than be spent, and people's TDS meters rise and they ditch the resin, and the cheap pre-filters will help avoid that. You can tell if this is the case, as if you get a bucket of cleaned water, and swill the resin in it vigorously, if you get extra life out of it that way, you know it was clogging.
Colour changing resin is a thing too if you've not used it before.
If you've room for it though, get a 200L blue barrel with a float valve on it, and any cheap 50GPD RO unit will then suffice. I used to have this set up with a 6 stage and used between 200 and 300L per week from the tank, it'd just replenish whenever you used it. Admittedly in Manc the water is very pure only around 50ppm, but I didn't have to change the filters or membrane in over 2 years. Only changed the DI resin once, but it was only a small vessel, and I was strict about having 0 ppm at all times.
