A lot to unpack here.... Let me help.
First, the 5900X will sit at 90C all day and be fine. It's a hard thing to fathom, but it's true. It's only going to get weirder with the 7000 series as they rev to 95C and sit there all day long. I would suggest you set the PBO to 85C max to give some wiggle room for spikes, and cap your wattage to something reasonable. An undervolt of -10 per core would be a good place to start. With that setup you should see max all core frequencies of 3.6 to 4.0 depending on which units the load is running on. For reference, I have a 5950X on a Noctua L12S.
Second, high temperatures in single core workloads are expected in Ryzen 5000 series. It's normal. Games, for the most part, only use a few primary threads which will trigger this style behavior. In your picture, Core 0, 1, and 4 are being hammered.
Third, Ncase M1 is basically a decade old design that is not a positive pressure case when you have an axial fan GPU cooler. You're dumping 350 watts of heat GPU into a case designed for 180 watt blower cooler. You need to exhaust that heat. Luckily you already have a decent setup for that. Which leads me to....
Fourth: Let's try this for your airflow. It won't be perfect, but it should help. Make the rear 92mm fan an intake, and have the center CPU fan as a pull configuration. Next flip the 120mm side fan to exhaust. That should help pull a lot of the heat out of the case. I don't recommend exhausting with the Corsair SF750 as it's already underpowered for the power spikes the RTX 3080 delivers. As such, make sure it's as cool as possible, and functioning as an intake.
