I've been reading around a bit, my brand new r5 3600 is also leaving something to be desired in terms of heat dissipation.
I need to do more tests later, but for now, in the delid of my micro I've seen that they solder the his with gallium, and this not only has a lower thermal conductivity than hard solder, but it turns out that the layer of gallium is very thick!!, a youtuber after delidding and applying liquid metal, and having cleaned the resin perfectly, had to sand the lower edge of the his so that it made contact with the die, in the same photos you can see what looks like several millimeters of gallium between die and his.
This would perfectly explain what we observe.
it seems that the new dies are more sensitive to pressure, which occurs with the expansions as well, and there they cure in health with a softer solder that accommodates the volume changes a bit, and with a thicker layer for the same purpose.
that's what I read earlier, and everything adds up, really.
For now I'm not going to touch anything, when I get back from my week of vacation I'll get a laser thermometer and I'll come to more empirical conclusions.
I have more than acceptable temperatures, but I can't get over the idea of having banned access to all the performance that's left because of a "thermal bottleneck".
If I discover something worthwhile I'll keep commenting.