Is it permissible to pray behind an imam who is not serene in his prayer, who chants the Qur’an in a way that makes it difficult to achieve Khushoo' (devotional humility) because of him, and there is no other mosque?

If you can pray behind the imam a valid prayer that fulfills all conditions and pillars, then the congregational prayer does not cease to be obligatory for you. Rather, pray with him even if you are uncomfortable, especially if there is no other mosque. It is recommended to continue advising him gently and wisely, and to seek assistance from other worshippers.