Hi kid,
thanks for the feedback!
I invite you to try to change the structure of the GxEPD source tree to your needs.
I know this source structure does not fit well with the capabilities of the Arduino IDE; it didn't with 1.6.x and still doesn't with 1.8.x.
The .cpp source file needs be included where the instance of the display class is created, because it is not automatically included in the build from a second level subdirectory.
However there should be no problem to pass a reference or pointer to the display instance to classes or methods in other files; in these only the header file needs be included to make the type/class of the instance known.
You could also consider to use one of the libraries GxEPD2_32, GxEPD2_AVR or GxEPD2, which have a simpler source structure.
Btw: why do you use the old-fashioned style of instance creation with copy assign operator?
I know the compiler optimizes this sequence of two creations, one assignment and one destruction, but C++ fans don't like to see this.