Hi Tim, from what I’ve read there isn’t an issue with water “going bad” after six months; it’s more that it loses its freshness, taste becomes kind of gross. And in particular if the storage containers are clear/translucent and are also exposed to sunlight, there might be some issue with leaching chemicals from the plastic containers into the water. However all that can be solved by filtering the water before drinking, and of course it would still be usable for non-potable activities. At our last house we had a 300 gallon cistern that held water for months. My approach is better dirty/old water than no water. Filter before use. But for stored drinking-ready water, I’d switch it out every six months.
Sounds like you’ve got great options for water. We have a spring nearby, too, and I love the water from it. I would recommend testing if you’re using it after any major weather event: a hurricane can create lots of damage to water/sewer systems and create ground disturbance that can cause issues with previously clean sources. We filtered even our spring water for several months after hurricane Maria.