As far as I'm aware there's little that can be done via BIOS / RMaster except setting an all-core frequency and voltage. Your best bet is, I guess, Clock Tuner for Ryzen 2.0 app. You can get it, for example, here: https://www.guru3d.com/files-details/clocktuner-for-ryzen-download.html. As far as I'm aware, it should allow per-core configuration of Ryzen 5000 series CPUs. But I really didn't bother with it, just set all my cores to 4.35GHz@1.181V and it runs smoothly unless tortured by something like LinX / Prime95. I also have a 5800X.
Officially, AMD considers temperatures of about 90°C within perfectly fine operation range, with critical being over 100°C (or even 105... I don't distinctly remember). Hence it boosts until this 90°C thermal ceiling and only then downclocks. But it doesn't mean it's actually OK to run it at that temps - you might be shortening your CPU lifespan from theoretical 7-10 years to 3-5 years, just outside official warranty support period, heh.
