Charging the battery slowly does not increase the life of the battery. If that were true, then users would be recommending 1 watt chargers or less
The phone automatically determines its own charge rate, even if the charger is 65 watts or more. If you use a 5 watt charger, the phone will charge more slowly than the Apple recommended 20 watt charger.
Make sure that you have Optimized Battery Charging enabled on your phone and you don't have to worry about things. Plug the charger into the phone and leave it connected all night, every night.