Coates hasnt expanded on the title character?
Let's see:
-Tchalla was the one who pushed for a council of wakandan leaders to make decisions that impact the wakandan people; not just one man.
-In doing so tchalla challenged the tradition and norms not only of the royal family but of the ancestral black panthers.
-Has made an effort to address all the missteps of his past including reconciling with his sister after leaving her to die and rekindling his love with his ex wife.
-Bringing together a group of heroes to defeat a God hellbent on destroying wakanda.
-Sending a crew of wakandan explorers into space that would eventually lead to an empire of wakandans that was the most feared and respected in the entire universe
-Been an inspiration to people from the future all while to trying to regain his memory and figure out his place in the world.
I'm not sure how anyone could be reading these titles and conclude that Coates has not expanded on the character. He is trying to show the struggles and duty he has to go through as man/king; I think he has done a terrific job imho.