Is it sinful to not have a beard or to do other makruh actions?

If a Muslim does a makruh action, like if they trim their beard short (according to the shafi school) is it haram that they enjoy the look of it or if someone copies their sheikh's style of beard (while knowing a longer beard is better and closer to the sunnah). What is the remedy for waswasa on fear of "being pleased with sin" / "approval of sin" or waswasa on"fear of istihlal". Any advice for someone who gets waswasa about intentions and obsessively thinks about permissibility and compulsively researches about matters.

It really depends on your personal understanding of the matter. Personally I believe that the recommendation to have long beards was meant to fulfill a purpose (to distinguish the Muslims from the non-Muslims) at a certain time and place (in the context of the early Islamic community in Medina). If the beard had been a crucial aspect of the Muslim identity, we would have expected to have many more hadiths recommending it.

So dealing with such matters depends on your personal hermeneutic approach to hadith. Personally I consider the Quran’s commands and prohibitions to be timeless, meaning that apply to all times. But as for hadith, they should be interpreted as having a dual aspect: one aspect is the Prophet’s PBUH reception of non-Quranic revelation (such as how to perform the prayer), while the second aspect is the Prophet’s PBUH personal approaches and solutions to the problems of his time.

So when the Prophet PBUH recommends that we let the beard grow, rather than considering it a divine command, we can consider it a recommendation that is not meant to overpower a Muslim person’s own will and preferences. A Muslim has the choice of either following the recommendation or doing something else that is more fitting to their own time and place.

The issues of makrūh (disliked) and mustaḥabb (recommended) are not commandments but things that are on an equal footing with a person’s cultural and personal context, meaning that if they have a reason to do otherwise, this is not a sin. It is left to their own logic and preferences to determine what to do. If these things were meant to be commands and prohibitions, they would have been expressed as such. But they are not.

The solution to your waswasa could be to dedicate yourself to the Quran and its commandments, making it the constitution of your life. As for the makrūh and mustaḥabb things, by considering them equals to your own choices and preferences (rather than considering them orders meant to control you), you can perhaps convince yourself not to be bothered by them. Of course there are different levels of makrūh, so I am not recommending just ignoring them all.

The Prophet PBUH had infinite respect for his Companions and allowed them their own freedom sometimes to even go against his wishes. Some Companions wore silk despite it being prohibited because they had a skin condition that made other types of clothing bothersome to them and the Prophet PBUH permitted it. Think of the Prophet PBUH as a kindly friend and mentor rather than a controlling judge over you. His commands and prohibitions cannot be ignored, but his recommendations can be treated as gentle and friendly advice.

At least, that is my approach. I am dedicated to following every letter of the Quran and every proven command and prohibition of the Prophet PBUH. This is the constitution of my Islam and enables me to feel fully “Islamic” in my identity despite the fact that I do not have a long beard.