Aesthetics is cultural and, therefore, learned.
I completely agree with this but also I don't think you touched on some really important aspects of what this means and some other interesting factors at play in the appreciation of art.
For this kind of discussion I usually like to divide the experience of art into 3 different facets: art (communication), aesthetics (evolutionary responses to environment), and culture (accumulated experience). This is just my very makeshift way of understanding it and I'd actually really like to hear other peoples' extrapolations.
Culture (accumulated experience) I define in basically the same way as you defined aesthetics above, but with the entirety of your life's experiences in there too. It's the reason why languages from people who subsisted off of fishing, trading, sailing etc. have more words for blues and whites compared to a language that comes from people who lived in a dense jungle, who'll have more words for greens. You notice more subtlety in things you're used to interacting with regularly and so have more appreciation for nuance. Somebody who has a lot of experience with the very rigid contemporary Japanese styles of bonsai will be able to enjoy it more because they know what to look for and what is considered "good". As you said, a baby can't tell the difference, and somebody who immerses themselves in their own bonsai culture, e.g. Walter Pall, won't have that same appreciation either.
Aesthetics (evolutionary responses to environment) is kind of similar but this I think of as the way our brains are innately programmed to respond to seeing eyes, animalistic shapes, response to smells of predators, seeing geometric shapes, etc. These are all things that aren't created through experience and just exist as passive background functions and are almost identical from person to person, regardless of where they're from. This affects how we interpret artificial shapes as compared to natural shapes, similarity to human and animal shapes, stuff like that, and can have a really big affect on what we see as unindoctrinated viewers of an artform.
Art (communication) is more a product of the interaction between the culture of both the artist and viewer than its own thing, but the communication can definitely be a prescribed message that is either understood by or lost on the viewer. In Japan, for instance, there are some really nuanced and complex conventions regarding display of bonsai trees that can be understood and appreciated by other people who understand them, but can be completely lost or misinterpreted by somebody who has no clue about Japanese culture.
Of course all of these bleed into and affect each other in complex ways but what you're talking about with "Being a self-learning person only takes you so far. There will always be knowledge gaps because there will inevitably be things that you don't know that you aren't even aware exist." is just saying 'my culture & art > your culture & art', which I don't think is a super strong argument. Yeah, there's no other bonsai culture & art in the world that's as nuanced and developed as Japanese bonsai culture & art, but that doesn't mean it's better. Just more expansive.