I'm not really an expert on HRV, I just trusted the validation study here:

https://www.researchgate.net/publication/315059917_Comparison_of_Heart_Rate_Variability_Recording_With_Smart_Phone_Photoplethysmographic_Polar_H7_Chest_Strap_and_Electrocardiogram_Methods

You could look at https://www.hrv4training.com/ to measure your heart rate variability. I believe it uses your phone camera and is scientifically validated. Should be more affordable.