Apple Support can actually test your iPhone's battery remotely if you don't want to make the trip to the Apple Store / Genius Bar.
To determine whether or not you might be wasting your time though, if the Battery Health / Maximum Capacity is in the mid to upper 80s or higher, your battery is doing just fine.
Apple will not replace a battery unless the Maximum Capacity has dropped under 80%
You do have the battery set to use Optimized Battery Charging with no charge limits, correct? And, you are connecting the power supply to the phone late at night and leaving it connected all night.....every night, correct?
More info here on the best way to charge the battery:
When to charge your iPhone or iPad - Apple Community