I honestly think your scene samples (@2000) are too high, you should see improvement almost immediately when you increase your material samples. Try 32 samples for your materials, and you may have to go higher (64 is usually my absolute max) if your results don’t get better.
Your material samples contribute to the overall quality, but if they’re set too low you’ll never get optimal results in a scenario like this. You should be able to render at lower scene samples if your material samples are high. Does that make sense?