Should I add two more 8GB RAM sticks or replace them with a new kit?

Yes, add two more 8GB sticks if you can get the same or very similar speed and timings. It’s cheaper, performance is nearly identical, and you avoid wasting the RAM you already own. Only replace if your motherboard has only two DIMM slots or you plan to go to 64GB. If you can’t match your current kit, buying a new 2x16GB kit and selling your old sticks is a clean alternative, but it’s more effort and money for no real gain.