Does the exterior of the case get physically very hot? If it does, then your problem is that the case can’t shed enough heat. If the case is mostly cooler, or only has a hot spot in the middle, then it’s an issue with getting heat from the die to the case.
I would be looking at something like a 100x100x3 copper shim, to help with not just moving heat straight from the die to the adjacent section of case, but also spreading the heat sideways. Heat pipes would be nice but 3mm is too thin for a DIY solution.
Case is cool all over, 3d printed a shroud for a 120mm fan to blow down on it, even the spot directly over the slug is cool, which makes me think it’s that blasted aluminum puck that’s holding me back.
If I forgo the case, a vapor chamber with heat pipes would be perfect. I guess I could just zip tie a cooler on there, but was hoping for something less jankey. Finding copper in the right size is my preference, but that stuff gets expensive quick.
Have tried thermal grizzly, arctic and noctua thermal pastes and a plethora of pads as well.
Polishing with finer and finer grit of sandpaper? I don’t want to used a chemical polish because it might stick around, right? Could use automotive sandpaper I guess… might give that a try.
I’ve never done it, but it’s something I’ve seen people do to the heat spreader on the CPU. And yes it would just be mechanical sanding and polishing with paper and paste.