Complex sentence: It was started in response to the success of Garo (which at that time was around its highest circulation), and as a way for Tezuka and other artists to showcase more avant-garde and experimental works in manga.
Split into simpler sentences: It was started in response to the success of Garo (which at that time was around its highest circulation). It also was started as a way for Tezuka and other artists to showcase more avant-garde and experimental works in manga.