Should I add another 8GB DDR5 stick to my existing one, or buy a 2x8GB kit?

Only add another stick if you can find the exact same model (same part number, timings, voltage). DDR5 is picky about mixing sticks—different brands or revisions can cause instability or slower speeds. If you can’t match it exactly, buy a 2x8GB kit instead; it’s safer and you can sell your old stick for about $20.