Thanks. Yes, an outline is a good idea. I have not worked with custom shaders much - the space background was the first try. Do you have some hints how an outline shader could be done?
https://github.com/GDQuest/godot-shaders/blob/master/godot/Shaders/outline2D_out...
Note the license on this and include it in your game if you implement it. You should pretty much just be able to copy it into your project and start using it - I've had good experience with this shader, and GDQuest's shader repo generally.