The Strongest Dull Prince’s Secret Battle for the Throne

[BlueDragoon]

Title: The Strongest Dull Prince’s Secret Battle for the Throne
Romaji: Saikyou Degarashi Ouji no An’yaku Teii Arasoi
Japanese: 最強出涸らし皇子の暗躍帝位争い

https://bookwalker.jp/series/216783/list/
https://ncode.syosetu.com/n2377fh/

Summary:
The protagonist is nicknamed 'the dull prince' due to his laziness and incompetence, or so he wants others to believe. This way he can ditch his responsibilities to go out and play around as a mysterious masked adventurer. But with the battle for the throne heating up he realises his loved ones will end up dead once one of his power-hungry siblings wins the title of crown prince/princess, so he decides to help his younger twin brother become the next emperor.
Nobody's going to be paying attention to him since he's so 'dull', so he can easily conduct schemes in the background. He also has the assistance of the empire's strongest adventurer - himself!

Impression:
It's similar to Altina in that there's a bunch of siblings fighting to become emperor, but this series focuses more on political maneuvering rather than battlefield tactics (although the characters do occasionally go out and fight things). The protagonist is interesting in that he willingly lowers his reputation for the sake of his own freedom, and because it's easier to help his brother that way.

There's currently 4 volumes and a manga adaptation being serialized.