I use a RO buddie 4 stage and it works very well though a bit slow at producing RODI
i also bought a extra carbon stage to add on along with eventually upgrading each section of it to a new bigger stage container when they run out
also there's the exact same or similar water pressure and tds meters on BRS you can get on amazon for cheaper

Good thoughts overall. Since your solenoid is NC your float switch will have to be set to NO if I'm reading what you're planning on doing correctly.

to make these jugs of de-ionized water more potable

To help with this remove the post DI filter. For drinking water there's no reason to have DI water. If you want to add back more minerals then search for a filter with calcite (calcium), corosex (magnesium), or both. Remember, after you install this filter your water will no longer have a TDS of 0, and that's a good thing.

For jug sterilization search for "sani-system". That's a food grade sanitizer made for RO and softener systems. If you're using it on a 5 gallon jug you'd use a tiny, tiny drop of this chemical.

Also for your TDS rather than going with a handheld meter I would go with an inline meter with dual connections so you can monitor your incoming and outgoing water continually https://www.amazon.com/HM-Digital-DM-1-Monitor-Accuracy/dp/B001EHAZGW