I've found a very quick method for farming quills, and used it to get my last 7,500 quills in about 15 minutes, and so I thought I'd share. Equip the tonics: Less checkmates, googly eyes, quillsplosion, and if you have the 4th slot unlocked then add quill magnet. but it should still work without.
Load up Chapter 3 Wild Web Woods - Wilder
The moment the level starts, enemies that run at you will be infinitely and rapidly spawning in, and despite the fact that googly eyes is active, enemies spawned from the initial spawn point have no googly eyes and will die in one hit. Each enemy drops 5 or 6 quills. You can get a rhythm going whilst jumping, and it can be done pretty mindlessly without any of them getting past you. If you take a hit and lose Laylee, you can just walk over the starting checkpoint to bank the quills you've earned, and then kill yourself and carry on. Just speed through the level once you have 40% of your desired quill total to profit from the 2.5x bonus upon level completion.