Thus far, it appears the essential for many are RODI units and ATO uni
Not surprising. Anyone that has ever dealt with the hassle of 5 gallon jugs being refilled at the LFS weekly or realizing just how much water evaporates out of your tank daily had their daily aquarium upkeep life drastically improved with ATOs and RODI systems.
I'd add another item to my list and that is a good pump used for moving water from one container to another or to the aquarium during water changes made the process way more efficient and easier.