I don't have the exact date for the next Tezuka Manga Contest. But often, such information is shared on social media accounts dedicated to manga or in newsletters from manga organizations. You could look there.
Knowledge competitions that often appeared in online novels generally referred to a kind of intellectual competition between the experts in the novel. In this kind of competition, participants had to answer a variety of complicated questions and score through voting or discussion. This kind of competition often appeared in novels as a way for characters to interact.
A well-known manga on this topic is 'Blue Giant'. It focuses on a character's passion for making manga and the challenges along the way. The story is engaging and gives a realistic portrayal of the manga industry.
The initial step in creating a manga is to determine the genre and theme. Decide if it's going to be action, romance, fantasy, or something else. Then, start fleshing out the basic story framework within that genre.
First, you need to have a clear story idea. Think about what kind of characters and plot you want to create. Then, start sketching out your characters and rough storyboard.
I haven't heard any news about Hajime Isayama creating another manga. Usually, such announcements would come through official channels or reliable sources, and there haven't been any of those yet.