We also have Siberian larch, it was significantly cheaper and will simply be completely replaced in X years. I prefer to buy multiple times. Every wood eventually wears out, including tropical woods... Our wood is only treated with a clear wood oil (no color pigments), as we also want it to gray out. However, we want to avoid getting splinters in our feet and the wood weathering too quickly.
Do you already know what your railing will look like? We still need one as well but haven't decided yet. We don't want it to look like too much wood / bulky (we have about 30sqm 6m*5m). But I also don't want prison bars made of metal.
Edit:
Ours is grooved; at first, we wanted it smooth too but decided against it because it might become slippery quickly when wet.