Why wait for the iPhone 12? The current iPhone already charges significantly faster over USB-C than USB-A, as do all iPads released in the past ~3 years.
There's really no reason to worry about it. Battery replacements (at least from Apple) are $70, and all modern phones have sophisticated battery management controllers.
Also, fast charging your battery more often could be better since you're not fully discharging and recharging your battery.