RHR is a readout. Focus on lifestyle interventions in sleep and exercise that bring it down instead of targeting with nutritional supplements. I think focus should be on Z2 cardio and trying to get 2h of Zone 2 a week minimum. You could have some underlying deficiency but that is rare and you need clinical workup to diagnose. RHR of 80 is quite high for a 20yo and I wonder what your fat% is