Should I add two extra 4GB RAM sticks instead of buying 2x8GB?

Yes, add the two extra 4GB sticks — it’s cheaper, works fine, and you won’t notice a performance difference vs. 2x8GB. Four sticks still run in dual channel, so you won’t lose speed. Just buy the same model or matching speed/timings. The only downside is using all four slots means upgrading beyond 16GB later requires replacing those 4GB sticks, but if 16GB is enough for this PC’s lifetime, save the money and hassle.