This thread is interesting but a lot of it seems to revolve around the theory of harmony. That’s all nice and fine, but it’s only one aspect of music out of many. For me as “Ambient” musician (i hate hate word) harmonic shifts are not very prominent at all. Also all this harmony theory for me is an expression of a certain bourgeois high-culture that doesn’t really represent me.
Spectral shifts are a much bigger part of my music. A track can be carried by an extremely simple folk-like chord progression, say Cmaj to Amin. That can be perfectly sufficient. All the rest happens in the shifts in overtones, transients, in tonal density, in the harshness or softness, the dynamics and all that. Maybe you can call it the Scelsi- or Dumitrescu- approach if you want to root that somewhere in high-culture.
I am not implying that I am special here, a lot of people approach things like that in modular, especially because placing harmonic shifts precisely can be quite a challenge in modular, so maybe it almost comes natural to do it like that. I am just saying that you don’t necessarily have to worry about modes and the circle of fifths too much in order to make “ambient music”. Sometimes this knowledge can even be in the way maybe.
Then again there’s artists like Tim Hecker who I know is very specific about notes and harmony. Different people different approaches.