Something I found helpful.

Start with 500ml of old developer (warmed to 70 or 80f) in the tray. With the lights on, drop a test strip in while you're setting other stuff up. See what happens to the test strip. Sprinkle a pinch of sodium sulfite on it, see what happens.

This way you can find out what is in the old developer, to some extent, before adding developer and water to full volume.

It's also a great way to test the infectious response of a known lithable paper. What makes the image explode on Forte or Foma isn't exactly the same for the Art 300, I bet. And congratulations on the new addiction.