I do think that spending money first on insulation and air sealing first will be more “worth it” than a stove insert, but that is true for anyone, not just someone in a moderately mild winter climate. We insulated and air sealed both homes where we lived prior to any fireplace upgrades.
Another question to consider is whether you are happy with the level to which you heat with your current heating setup. When we installed an insert into the finished walkout basement of our Virginia home, it was because it was unpleasantly cool down there when the upstairs thermostat was set to sixty-eight, which was as high as we set it. The basement became everybody’s favorite place to be at that point during winter.
In Texas with a much warmer winter climate we found ourselves very cold in our house because it was set up to be air conditioned more than heated. It was built on an uninsulated slab with ceramic tile on top in the area where we spent most time during the day. It had higher than average ceilings throughout. The heating vents for the gas furnace were in the ceiling, and the heat flowed across the ceiling and up the stairs before we felt any real benefit, and we weren’t willing to turn the thermostat up high enough to make it truly comfortable. It was a few years before we got our wood insert installed, and we did a lot of energy efficiency upgrades first, but we were so thankful when we finally had wood heat because we were just simply so much warmer in our open family room/kitchen/dining room area. We had a tall chimney, but we burned regularly when overnight temperatures where in the forties and thirties [and sometimes twenties and teens or even single digits] and daytime temperatures were in the fifties or even sixties if it wasn’t sunny. We didn’t keep a fire all day in warmer daytime temperatures, but overnight fires with morning stokings were common.
I really can’t speak to your payback period, but I just wanted to mention that there may be things to consider other than just the monetary return on investment. In our cases we were able to save money on our heating bills and feel more comfortable at the same time. We were particularly glad to have the wood insert during the winter storm in Texas in February 2021. We did not suffer the rolling blackouts that others did because of our proximity to a fire station, but we did lose power at points just due to the storm. When our electricity was on, we turned down our furnaces to conserve fuel and pushed the stove much harder than normal for us, probably closer to normal for most woodburners in cold climates.
Just because North Carolina isn’t as cold as some other places doesn’t mean it isn’t cold enough to realize a benefit. If you haven’t looked into insulating and air sealing, though, it would be wise to do that first. It will help your cooling needs, too.
And yes, pine and poplar are safe to burn if they are dry. I prefer pine over poplar just because poplar leaves a lot of ash.. We burned a lot of cedar [truly a juniper] in Texas. It was great wood. Our live oak was so dense that the cedar was useful for helping to burn down the coals, so we mixed or alternated loads a lot. When we lived in Virginia, we burned a lot of red oak, but it took a lot of time to season compared to pine or poplar, so we kept those on hand, too.