Battery - how to charge

somebody correct me if i'm wrong, but for lithium polymer batteries it's recommended that you stay between 20% and 80% charge.

I think that's correct.

It would be great if Garmin implemented limit like Apple Watch. I don’t need 100 all the time. Add an option for health preservation 

You will get about 300 charging cycles.  Dice it up however you wish.

This would mean 4-5 years for me with AOD, and after that it starts to get worse and not fail completely. 
Never had enough discipline (or to much faulty hardware) to stick to a Garmin watch that long. I think the longest was Fenix 3 to Forerunner 945.

Edit: checked it out: this was 4 years long

As soon as the new model is released, I buy it and then sell the old one.  I do this for most of my equipment.

Hello, my Dear Friends ,
I wouldn't exaggerate with the form/way of charging .
For many , many years , I have been discharging phones or watches to zero... and charging them all night to 100 .

I have not noticed any decrease in battery efficiency .
What's more , I sell phones after about 2 years and my battery efficiency is at 99.5-99.7% .

I don't see the slightest justification for charging: 20%-80%  .
The battery has its own safeguards and the phone/watch also has its own , which prevent overcharging .
I emphasize - I have been charging this way for many / a dozen years .
Best Regards - Marcin .

So for you its not important how long a divice can be used and for the buyer of your old watch its not your problem. Very farsighted and sustainably thought. Well done.

You completely misunderstood what I wrote .

It is precisely out of concern for potential buyers or recipients , who are often my close friends , that I take care of the battery in my watch or phone . That is why I wrote that before two years have passed , the battery has over 99% of its condition .

I wrote earlier about the way I have been charging the battery for a dozen or so years . Well done .

He/she was not replying to you. Well done.