Is it permissible to trim the mustache out of personal preference and leave the rest for the sake of reward?

It is not permissible for a Muslim to make his religion subservient to his whims, rather his whims must be subservient to what Allah and His Messenger have commanded; as Allah the Almighty says: "And who is more astray than one who follows his own lusts, without guidance from Allah?" and the Prophet (peace be upon him) said: "By Him in Whose Hand is my soul, none of you believes until his desires are in accordance with what I have brought."

Acts of worship are based on divine law and adherence, not on whims and innovation. Islam is built upon the Oneness of Allah and His worship according to what He has legislated through His Messenger (peace be upon him), not according to whims and innovations, as Allah the Almighty says: "Or have they partners who legislated for them of religion what Allah did not permit?" And every instance where Allah mentions whims, it is in the context of dispraising them and those who follow them.