The first two binaries / sliding scales that sprung to mind were electronic vs acoustic and clarity vs blurriness. Where each of those is probably conflating a few other ideas, so "acoustic" covers both the sound quality of acoustic instruments but also the "feel" of live playing, and "clarity" simultaneously refers to clarity of tone, pitch and music gesture.
I thought a bit about soothing vs unsettling but I don't thing that really works as a sliding scale - something can have soothing and unsettling elements in tension and be quite different from something that's just sort-of neutral. You maybe want something more like a mood wheel for that, with independent axes for, say, calm, ecstatic, melancholic, unsettling, cinematic, playful etc.
clarity vs blurriness is great. definitely an essential addition. i agree soothing vs unsettling can be so multifaceted, the way i tend to sort of consolidate disparate emotional impressions is by asking: would i want to
live there, in a place with that atmosphere? sometime i find that the music has an unsettling quality but it's so lush that i'd still say yes.
acoustic vs electronic is probably getting to the heart of what this is about, for me. thinking about it not so much in terms of the nature of the sounds produced as what they evoke. so acoustic meaning, like you said, "live playing"—music that sounds like a dude playing an instrument; electronic meaning audio animation—music that sounds like an imaginary environment. in more formal terms you might say that acoustic is all about traditional melody and harmony whereas audio animation makes greater use of timbre and space. (hopefully i'm riffing off what you said and not going
completely in a different direction...)
it seems like ambient musicians often approach what they're doing on very traditional terms. they see themselves as a performer when their music would be more interesting if they saw themselves as, idk, a movie director or a mage something. so imo, "advanced" ambient moves beyond that.
"audio animation" is a great way of looking at music generally (especially electronic but not exclusively)
ambient, though, feels like it might be one kind of music where it has less traction - simply because the music is not very animated in the sense of motion and liveliness.
in a spiritual sense much of it is aspiring to the grace of the inanimate
probably ambient is more like (at its worse) a screensaver and (at its best) a mandala
also certainly kinds of visual art - Rothko... Monet's Waterlilies was a big one for Eno
I also often think of things like mobiles - the Calder kind but also the type that gyrate very slowly above a child's crib.
i suppose music doesn't need to be
that lively to work as audio animation? evoking relaxing, graceful, somnolent motions like the migration of clouds, fog rolling in, or yeah on the really low end a screensaver, is fair game surely.
but you're right, my main interest is the "edge cases": to borrow droid's dichotomy, stuff that's unusually kinetic for ashram music and stuff that's unusually lush for academy music. an example of "advanced ambient" on the former end would be like tetsu inoue, who started off very culturally aligned with all the music for nodding off after raves stuff but was always as brilliant an audio animator as the best electroacoustic composers. an example on the latter end would be later francois bayle, who by the 90s and 00s had left behind the shrill tinnitus synths he'd used in the early 70s and was using the same sort of lush "cyber gaia" synth pads appearing in ambient around the same time, albeit in a much more active way.
imo this is a great synthesis of ambient and electroacoustic values into lush audio animation. a sort of parmegiani-esque compostional structure of having sounds moving violently in and out of a more stable "continuum" that subtly changes in their wake. evokes being in a wide open space on another planet, peripheral ufos careening through the atmosphere.