Possibly a good way to make your resin last longer is to divert the RO water for at least 5 minutes on start up; there is a "spike" in TDS on start up which prematurely depletes the resin in your vessels downstream of the RO kit.
Easiest way to do this is buy a "normally closed" in line solenoid ( eg.240Volt) and install it on the line between RO & resin vessels. A simple 240Volt timer will do the rest, turn the solenoid on, then your water & the solenoid will automatically send high tds RO water to drain, then resume filling your holding tank after 5 minutes or whatever you set the timer to. If 240 Volt doesn't appeal, you can get low voltage solenoids too. (eg 12 volt, 24Volt etc). This is how the most expensive window cleaning machines maximise resin life.