Great Build!! Im jealous
m sure you have it covered, but I just want to point out in case.......your 25gal freshwater top off resevoir is pretty large. If you have an issue with the overflow drain lines getting plugged, then the return pump chamber will empty into the tank, which will trigger the ATO, which will keep trying to fill the return pump chamber but it wont be able to because the return pump will keep pumping water into the tank......The tank will eventually overflow and equally bad have a huge salinity drop because of the large volume of top off water that would be dumped into the tank.
maybe your ATO has some limiting factors on how much it will pump per day or something to avoid the above issue, but still I think I would not feel comfortable trusting the electronics.