Would a hard tail 29er be a good starter bike for road and light trails?
Short answer yes, a hard tail 29 would be a good fit for what you have described. And it sounds like you already have a fun FS bike and a road bike so it only makes sense. The equation N + 1 comes to mind, where N is the number of bikes you currently own.

29ers roll like nobody's business, they don't accelerate like a smaller tire but the roll over is awesome. For longer 20-50 mile rides a hard tail would not hold you back either especially if your using a fork with speed lock out and smooth tires to reduce spent energy. It won't keep up like a road bike but it won't be completely unbearable either. But a road bike taken on a trail could get interesting quick for an amateur rider.

Sounds to me like you know what you want and have narrowed it down pretty good on your own. I say go for it though.