Hiroya Oku likely starts with a concept or story idea. Then, he sketches out the characters and plotlines. After that, he refines the details and adds depth to the story through continuous revisions.
It depends on your taste. If you like superhero comics with complex characters and exciting plots, then you'll likely think it's good. The world-building and character development are quite impressive.
The Invincible comic has indeed come to an end. The story reached its conclusion after a successful run. Fans have had to say goodbye to this beloved series.