Is not trimming the mustache except for what exceeds the upper lip only, while leaving its sides, in accordance with the Prophet's (PBUH) saying: "He who does not take from his mustache is not of me," and does this entail sin, especially given the questioner's fondness for this appearance?

The hadith you referred to was narrated by Imam Ahmad, Al-Tirmidhi, and others, and was authenticated by Al-Albani and others. Trimming the mustache is a matter of consensus, being a Sunnah. The method of trimming the mustache is to cut it until the edge of the lip becomes visible, but not to shave it from its root. As for the sides of the mustache, scholars have differed concerning them. In Fatwa No. 38307, we mentioned the preponderant opinion that they are part of the mustache, but there is no harm in leaving them, because they do not cover the mouth, nor do food residues remain in them. Therefore, there is no blame upon you for what you are doing.