Before this does get locked.....I will chime in that I think the objectivity vs subjectivity argument is fascinating. I just end up never bringing it up because I guess I’m one of the extremely few people who never gets tired of philosophizing about it. But....whatever.
I actually had a reply ready to be posted that was fairly long, and right as I hit the 'Post' button, I got the notification that the thread was locked. I honestly thought that was a shame, because like you I find the subject to be absolutely fascinating. I'm have a Master's Degree in Musicology and I'm a music teacher in high school, so it's discussions like these that I can absolutely dive into.
I do not have any music degrees, but I did study philosophy, so I get into this somewhat on the other end.
My view has some similarities to yours. All the arts have objective standards of value, but those standards are based on what that art is and what purpose it has. But these are very hard questions to answer and thus make the task of identifying those objective standards a tough one.
I think the easiest art form to formulate a standard for is literature, and this is because literature is already presented in the form of words and concepts, whereas music and the visual arts are initially perceptual, and so there is the additional step of conceptualizing things that are merely seen or heard. And I think there are a number of well-defined standards in literature, some widely accepted, many of them not. They ought to be defined in this way: literature has a certain nature and purpose, so we need a certain sort of work of that nature to best achieve the purpose (which I have reasons to believe is, for all arts: to concretize abstract ideas for human contemplation and enjoyment).
The key facts here are that objective literary standards exist, but are very challenging to figure out—even for people who study literature intensely. So they are all the more difficult for laymen. Where, then are literary laymen, if they don't have a strong grasp on the aesthetic issues here? They might be able to make approximate judgments (such as identifying the better book out of a pair where the difference is extreme), but for the most part, they are in a situation where they have to treat literary choices (above a certain obvious baseline) as subjective. They might like Dostoevsky better than Tolstoy, but they are not really in a position to argue that Dostoevsky is objectively superior. So they would have to think of their preference for Dostoevsky over Tolstoy as subjective—though it would still be a very good exercise to identify some specific elements about him that they prefer over elements in Tolstoy.
I believe the situation is quite similar in the other arts—I have certainly seen no reason to believe that objective merit exists in one art form but not the others. But, as I said above, the challenge is much greater in the other arts, because they don't come pre-equipped with a conceptual vocabulary—analysts have to make conceptual identifications of merely perceptual (visual or audio) elements. For example, "characterization" already conceptual to the literary critic, because they are told about the characters in conceptual terms, whereas "lighting" is something that the student of the visual arts has to take from visual stimuli and put into words.
Because of this additional challenge, objective knowledge is, I believe, much harder to accomplish in the other arts than in literature—though I believe it is still possible. And, once again, someone who is not well-studied in the art form in question and equipped with a conceptual vocabulary is in the position of having to treat those arts, above an obvious baseline (e.g., Rembrandt is better than a first-year art student), as subjective. Though it is still a good exercise, of course, to identify one's reasons for preferring one work to another.
Music presents a special challenge over and above the challenge of the visual arts. This is because in music there are no clear-cut concrete objects, and thus much less material to devise a conceptual vocabulary. For a novel, we can talk about how a character's words and actions have a certain effect. For a painting, we can talk about how the items in the images and how they are depicted have a certain effect (for example, we can talk about the positioning, posture, facial expression and lighting of human figures). And our ability to do this makes it possible to evaluate how well its effect accords with its purpose and how well it achieves this effect. But with music, we have no concrete objects.
Music doesn't present us with a courageous man, or a laughing girl, or a ship on the horizon. Music hits us on a level we understand much less clearly than the literary or visual arts. If we see a painting of a cheerful boy, we can talk about how that makes us happy and how they artist achieved that—because the artist is showing us some object that, if we saw that object in real life, it would make us happy. If we hear a cheerful song, we have much, much less idea what is going on that causes us to be cheerful. We just don't have the ability to describe how sounds actually work in our mind to achieve a certain effect.
This is why my inclination is this: Until and unless scholars are able to describe the means by which music acts on our mind to achieve a particular effect, I don't believe that it is possible to objectively judge the merits of music. I think we can guess at what objective judgments would be likely to be if we were able to perform them (e.g., my bet is that we probably would find Beethoven to be one of the objectively best musicians), but these are just informed guesses. Without the ability to go deep into what music tells our mind and how it affects us, I think we are all in the position of the laymen in the other arts: we can do no better than informed guesses and subjective preferences (which again does not mean that we should not try to identify those elements of certain compositions that we like and don't like, which is exactly how I approach writing about music in things like roulette writeups).