I'm in much the same boat, but using Valcool 700, and have similarly low hours but long storage times on my coolant.
If it doesn't smell, the micro preservative system is intact (there would be other indications if it had failed, too). Since you're watching the concentration (RI), that's staying in range, and I think you'd see a physical change if the microemulsion degraded seriously. I'd kinda like to know that the pH hadn't changed, too (test strip is good enough). About the only worry would be that there might be some hydrolysis or other degradation in the anti-corrosion system. I suspect that you can get 24+ months if you keep an eye on it...but I'd be conservative here.
My rule of thumb is to do just what you are: check everything, adjust as needed, and keep going- until I hit 12 months or so. I've made it a practice to flush and replace every year. I'm pretty sure I'm recycling good coolant. But, given the relative cost of a gallon of coolant concentrate and a new machine, I'm going to stick with 12 months or so as an expiration date on coolant. I won't go nuts if I'm a month late, or early, changing it. And, btw, I write the date of the last change on a hang tag on the tank, and in the machine log.