Here's a small update, I stuck a little more PE tape to the glass around the LEDs so that the light doesn't dazzle so much. I also stapled a few resistors between the two layers of felt and attached them to the 12 volt rail of the power supply. This definitely allows you to see a lot more (especially on camera) now, so thanks for all the suggestions!

What is still missing is an extraction voltage of a couple hundred to maybe a thousand volts to extract all the extra electrons that are left over after the ionisation. That'll definitely help with visibility/contrast of the traces and how long it takes for them to clear up again. But I don't know if I'll do that yet. For this I would have to build a large frame inside the glass jar and get an high-voltage PSU module.