It is normal for the battery health to drop 1% per month on average over a couple of years. This is just an average and you may see that drop 4% in one month and nothing in another.
All you can do is make sure you are using Optimized Charging in Settings and charge your phone every night. The phone will learn when you normally take it off the charger and will only charge it to 80% until just before you need it when it will charge to the full 100%. This does take a regular schedule to work correctly, but is the best way to get the most out of your battery while still having a full charge for the day.
You can expect to replace the battery after the health drops below 80%. The cost for Apple to replace it is only $89(US) and not worth the anxiety or changing the way you use the device just to get a couple more months out of it. Some user will attempt to manage every aspect of their usage by shutting off Bluetooth, Wifi, or Cellular Data when not used or monitor the charge on the battery and stop when it reaches 80%, but in my opinion, it is not worth the effort.