Keep in mind that an RO membrane is a percentage game, so if your water source is low in problem materials like phosphorus, you might be able to get by with just RO and not RODI.
Do you have any idea what's in your source water? That might help determine if you can continue doing without DI.
Keep up the experimentation! We aren't being the best stewards of the earth's resources, pumping oil out of the ground to make the plastic for the DI resin, using it once and then sending it off to the landfill.
In no way am I saying that everyone should stop using DI resin - depending on what's in your source water it might be an absolute necessity!
As usual, your milage may vary....
Tim