if you have access to windows, you can use optiPNG for lossless compression, and if you don't I'm willing to do it for you! I tested it on various arts of mine and saw no visual difference but a 5-15% decrease in size
if that's not enough you may need to use another tool to reduce the number of colors first and I can hunt for one that doesn't do it as aggressively as most that show up first on a search