So while I appreciate a good clean crumb tray, flashback is not caused by junky trays. It’s the laser reflecting off the metal of the honeycomb, especially when the settings are overpowered. Even a clean tray will flash back if you’re overpowered. You might be hitting your materials a little too hard, might be time for a material test to really dial things in.
I’m surprised that you can’t clean it with the soaking/powerwashing tricks, I get lots of resin from plywoods etc and I tend to get good results. What are you cutting that leaves such a tough residue?