The 20 watt charger (or more correctly power source) is not bad for the battery, because the actual “charger” is in the phone, and it monitors the temperature in the phone to keep it within safe limits. And it will stop charging at 80% to allow the phone to cool down, then resume at a lower charge rate once the temperature has stabilized.
However, there is nothing wrong with using a 5 or 12 watt USB-A charger, and, if you charge overnight (as you should) there is no need to use a fast charger. The absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates.