It is a standard programming practice that, if the same operation needs to be done in more than one place, you write a "sub-routine" to do it, and call that "sub-routine" at each and every point that the thing needs to be done.

This is the kind of approach that you need to adopt: whether it's a button or a serial command that invokes it, the action is the same - so make it into a "sub-routine"

In the 'C' programming language, the concept of the "sub-routine" is implemented by writing a function.

This also means that your serial port handling can be (almost) entirely independent of what actions are to be performed - as they are all just "hidden" within the functions that get called.
Therefore you can look at any serial port code for examples...