iphone's battery health issue

Your battery is just fine. On average, expect a decline of 1% about every 60 days or so.

The decline will not be linear. The battery might go a few months with no decline, then suddenly decline 2%. Normal.

You will drive yourself crazy if you are checking the battery all the time.

All that you really need to do is replace the battery when the health is down to 80%.

 Batteries, like people are individuals. No two people age exactly the same and batteries don’t age the same either. The exact usage, chemical makeup, storage conditions, exposure to heat and cold and about a dozen other factors mean that every battery ages differently. Battery aging is not a linear (straight line) and degrades at different rates as the chemicals in the battery age and other factors mentioned above. Apple expects your battery health to be at 80% after 24 months. If your battery health misses that expectation, contact Apple Support.

Please review the Apple Support articles on battery optimization. It contains the best  current practices for battery charging.

iPhone Battery and Performance - Apple Support

Batteries - Maximizing Performance - Apple

About the battery and performance of iPhone 11 and later - Apple Support