Repasted the CPU (KRYONAUT Thermal Grizzly) - I'd spread a thin layer all over, but when I lifted the heatsink to check, it was obvious the paste was too thin in some spots. Added a little more and reseated the cooler.
Ran HWInfo and immediately saw CPU idle temps of 32°C, so an instant improvement.
I've double checked the Windows power plan is on the Ryzen optimization setting and set Gigabyte's fan software to an aggressive curve.
Just running Hellblade: Senua"s Sacrifice (4k, all settings on Ultra) to check performance - GPU is at 65°C and CPU at 53°C.
I'll play a bit and check again - but a far better result than last night.