General septic system/cesspool/sand mound tips:
1. For system longevity, minimize waste water directed into the system. This cannot be over emphasized.
2. Pump regularly, based on system size and number of household members. Yearly is overkill, but can provide good mental insurance for about $250 a year if you want. Average is about every 2-3 years. Better yet, rather than just pumping, have system checked and cleaned. This involves a digout of access, inspection, cleaning baffles, stirring of the soup and pumping. Just sticking the pump hose down the system will not remove all solids.
3. No need to use any product. Plenty of evidence to support this, but you'll find opinions to the contrary.
4. Never drive over system. Never cover. Evaporation is main source of water removal.
5. Minimize use of obvious bacteria killing products, such as bleach, cleaning and painting chemicals.
6. Remind guests/family you're on septic/cesspool.
7. Primary source of drainfield clogs is suspended particles, such as what your clothes washer puts out. A fliter such as the Filtrol 160 can address this.
8. Make your next washer a front load, like a Maytag Neptune. Reduces water use by thousands of gallons per year.
9. Save for system replacement/maintenance. The system will eventually fail. I've seen systems 5-8 years old fail, then again, I know of cesspools working well after 100+ years. It just depends.
Given all of the above,
RO/
DI and tank wastewater would be better dumped elsewhere, but in the amounts you're talking, it's not that big of a deal. I'd just accelerate the pumping schedule a bit for a comfort level. If the water's easy to dump elsewhere, then do that some to all of the time. Just don't dump over the system or above your well if you have one. On-site waste water systems can be cheaper and process output better than public sewer if properly used/cared for. Good luck.