65C my *ss. Yeah 80C is normal for 5900X with PBO using NH-D15. Mine runs lower, but I seriously doubt you have my airflow as it is LOUD due to me having only high rev (3000+ RPM) IPC Noctua fans.
You can delay the rev up/down in bios to mitigate the annoying sound of them revving up and down. You can also reduce max RPM@temp if you connected it correctly to PWM.
If you’re mainly annoyed by the rev up and down over and over, it’s the delay you need to increase.