A 30 TDS out of an RO membrane is pretty high. You might be starting out with really high TDS tap water. An RO membrane should have a 98% rejection rate. So 100 TDS source water will yield 2 TDS post membrane. I think that your current system really is not up to par. The most expensive supply in an RODI system is the deionization resin so you want to lower the consumption as much as possible. There are a fair number of venders selling decent RO systems and most of the starter systems are reasonably priced.
For most reefers, getting an RODI system makes sense. 1) You get 0 TDS water. That pretty much eliminates problems of heavy metals and contaminants in your source water. 2) You know the quality of your water. If you buy from a store, you really do not know their quality control. 3) You do not have to go to the store, buy water, and haul it back home. 4) It is easy to have a tub of water and a tub mixed up salt water on hand. These things come in handy. 5) If your system is of any size, your savings on the production of the water will pay for you system in pretty short order.
