Looking at my progress I have to say I am quite satisfied with the content pipeline I have established for the game:
Spine and LibGDX are going hand in hand out of the box. The ability of Spine to give names to bones and look them up in the game makes it easy to denote, e.g. a barrel (that is rotated towards the firing direction)or a barrel-exit (that is used to spawn bullets) that can be easily accessed and used in the game code.
When I add a new enemy, I just name a barrel in spine, give it an idle animation and the (still rudimentary) AI moves it from A to B (B is your home planet) and lets it fire. However, enemies need physics (mainly for accurate collision detection) and all I need for that is a png image and Aurelion Ribon's Physics Body Editor.
So beware: The next enemy type is always just a few clicks away. :)
To round this up, particle effects are easily added with LibGDX's particle editor, and I added a computer voice to comment on whats happening ('entering gravity field', etc.) via the free version of this text to speech software.