Complex sentence: They started working on another album and MY CYLINDERS was released on 25 March 2015 with Hiromitsu already an official member by then, proving to be a worthy replacement to take over Taisuke's place.
Split into simpler sentences: They started working on another album and MY CYLINDERS was released on 25 March 2015 with Hiromitsu already an official member by then. He proved to be a worthy replacement to take over Taisuke's place.