Garmin should be looking at ways to give more accurate results.

HRV is an absolute value of the HR Variability, and it changes naturally when you are ill. There is no need telling Garmin that you are ill. If the HRV is lowered by the illness (and usually it is), the watch detects it automatically.

Yet, it doesn't. I use the Elite app and Polar strap for a sympathetic/parasympathetic morning reading. I know it is spot on because when I am ill you can physically see it on the chart, yet my Garmin does not show the same field. The Garmin in fact is saying Im balanced, ready to train, when I am so physically drained I can barely move. The Garmin is registering my stress level while sleeping that does show I am ill (because my sleep hygene is perfect, so illness is the only thing left to look to). Because I am cross checking I can see the Garmin is missing out on a piece of vital information, it is considering a baseline of sickness to be balanced, which it is not. 

You can use the strap with the watch, when you want more accurate HRV data. The optical wrist HR sensor has indeed its limits, and is more influenced by noise signals caused for example by movements.

If you decide to use the strap to take the overnight HRV, you'd have to disable the wrist sensor, because otherwise the watch continues to use the internal sensor, when it is available and no activity is being recorded.

Interestingly it senses naps several times each day where none have happened. I'm not immobile, I am usually reading and or waiting on medications kicking in, or resting a joint.

Even during my 2 weeks of covid my HRV did reflect this for only three days when the fever was at its peak.
4 Weeks of pause - When starting again garmin did not take all of this into account and immediately tried to bring me back to high intensity workouts where only base level training would have made sense to recover from all of this.