People get all crazy about these silly battery health numbers. The only thing about health that you need to know is that Apple won't replace the battery for no charge unless it dips to <= 80% before the warranty period expires. I can also be useful when buying a used phone to know the current state of the battery.
You need to replace your battery when its capacity no longer accommodates and serves your life style. This will be different for different people. My battery health is 80%. I work from home in a home office environment where my phone can spend sufficient time on a charger. That suits my needs. If I were out and mobile and away from a charger most of the time, I would need to get it replaced because it does discharge quickly.
In short, don't worry about a declining battery health number, it's going to decline as the battery ages and the decline may not be linear. Just use your phone and when it no longer sustains a charge long enough to suit your needs, replace it.
If you one who updates every year or two, it's not even relevant. If you keep your phone for a long time, you're probably going to have to replace the battery, period.
I think one of the worst things Apple has done is add that stupid battery life stat to the phone. Too many people needlessly obsess over it.
Best of luck!!!