What can I do to regulate iPhone charging rate (make it charge slower)?
Fast charging your phone via a MacBook is not going to "damage" your battery anymore than other kinds of charging. Yes, high temperature can affect the battery, but a temperature of 38 degrees C is not a problem in charging your phone.
There's no need to limit the charging rate beyond what the iPhone does automatically by itself.
Also there's no setting in macOS that will allow you to limit the power supplied to the phone charging through a USB port (besides limiting it to 0 by turning the port off completely).