Was hoping you were going to say MSI which you would have probably had Kombo Strike to use.
Im using Kombo Strike 3 which sets -30 on all cores and sets all the power limits too, but along with this I have a negative offset of 0.1000v.. core voltage is just Auto with PBO set to Auto.
Before I had Kombo Strike I tried these settings I picked up from Reddit a while back, tested them all with -30 curve all core with the same negative Offset/auto PBO/Voltage I use now and they all seemed stable with my chip.
Have you tried -30 on all cores with an -Offset? I've noticed on my chip that -0.1000v runs much cooler than -0.0875v Offset.. temp difference with these Offsets running CPU-Z/CB23 is around 5c cooler.
Maybe try similar settings to above but dont touch the CPU voltage and leave that on Auto, use a negative Offset like above with PBO also on Auto.
Using -0.1000v my max gaming temps are usually low 60's and CPU-Z is around 64c with CB23 around 70c.. all core is 4450Mhz, I even see this when streaming with x264 encoder. My ambient temps are similar to yours too btw, I never have my thermostat above 21c lol
Hope you get it figured out as Im sure somebody has said before that all the 5800X3D chips are binned so silicon lottery cant be that bad