A couple things. RO unit is for cleaning the water of salts and metals. (You know that). What makes RO valuable is the fact that it allows you to know exactly what is being fed to your plants.
I never left my wastewater valve open all the time. It was explained to me that you used this waste line to flush the unit occasionally. With this valve closed I was able to filter water to less than .1ppm. Maybe I am wrong but, that is how I used mine.
Filters should last way beyond 100 gallons.
Finally; You said you did not need much; So you won’t waste much; In my estimation.
Happy growing! lw