Whats the TDS before Resin mate, and also is it a HF4 or HF5?(low pressure membrane or high pressure)
If you have a HF4 you'll need a booster pump as they'll need approx 60/70psi +
This will increase your production & lower your outfoing TDS (meaning you use less resin)
Check this out if you need a booster-
https://www.machinemart.co.uk/shop/product/details/cbm240e-1in-multi-stage-230v-booster-pumHow much water do you produce a week? Changing prefilters every 2/3 months is about normal, chsnging resin depends in how big your D.I is...?(i've got a 25litre D.I, resin last about a year producing around 2,500-3,000 litres a week with ingoing at 6)
Also have you got a gate valve on your r.o?(tap on waste?)
Adjust it to your preference...
Lower tds but get more waste and slower production
Higher tds but faster production, less waste