The themes in this novel are quite diverse. There's the theme of adapting to change. Arthur is thrown into a completely different world, and he has to adapt. There's also a sense of justice in the story. He witnesses different forms of injustice and has to decide whether and how to fight against them. And of course, the theme of love is also present, whether it's platonic or romantic love, which affects his actions and decisions.
Arthur himself is very interesting. His character development from a powerful figure in his past life to someone starting anew in this new world is captivating.
The Beginning After the End manga has indeed finished. It provided a satisfying conclusion to the characters' arcs and the overall narrative. Maybe there'll be spin-offs or related projects in the future, but for now, the main story is complete.