Guidence for implementing a custom MoveGroupInterface

[MXS13]

Greetings. Since the original MoveGroupInterface only exposes some rudimentary functionalities of MoveGroup I want to ask for guidance in implementing a more tailored interface.
Afaik MoveGroup is a node which sets up all things required for motion planning (planners, robot information, ect.) and exposes services which the original MoveGroupInterface uses for its functionality. The true functionality is inside a private class called MoveGroupInterfaceImpl (which also contains important functionalities not exposed by the interface).
So is there a good way to start e.g. a description of services the MoveGroup node exposes?

For my use-case I have to plan with different planning groups, want to visualize the planned path for debugging and some more functionalities the original interface doesn't support/expose.
I would be grateful for any information. In my humble opinion, information on this topic is really sparse and hard to find, as soon as you want to plan in code and not Rviz (as you do eventually).