All it said was that spotify sells stereo songs and their app will play stereo songs. You can only play stereo audio sources in all 5 speakers if you enable realtek audio console's speaker fill or the equivalent stereo upmix in sonic studio 3.
To be honest, I can pretty much guarantee that it probably isn't an asus technical problem.
The problem is that you did a fresh OS installation, didn't know about sonic studio 3's upmix function, thought that there was something wrong with your computer, thought that there is something wrong with your realtek audio console app, and then you listened to the wrong people with absolutely the wrong "fixes" --- such as installing an ancient 2017 audio driver from the realtek's web site.
Of course, you didn't tell asus support that you tried all those crazy fixes, so they are not going to give you any useful advice.
Here is another guy (Flavberg) posted today that didn't understand why his asus realtek audio console didn't have loudness equalization. So the simple explanation was that realtek audio console's loudness equalization got moved to sonic studio 3's "smart volume" function. Fortunately for him, he didn't try any of the crazy fixes. So he turned it on and it worked.
https://www.tenforums.com/sound-audio/182081-latest-realtek-hd-audio-driver-version-3-a-70.html
If you google the internet on people complaining that why their realtek audio driver doesn't have the 10 band EQ (which is much more visible and vocal complaint on the internet), you will see all the absolute wild "fixes". But the simplest explanation has been around for over 10 years. If you have a computer with premium audio package, then the 10 band EQ (and other audio "stuff") gets moved to the premium app. See firebred's windows 7 computer back in 2012 with soundblaster app which has the 10 band EQ and his realtek hd audio manager doesn't have the 10 band EQ.
https://www.sevenforums.com/sound-audio/252007-realtek-hd-audio-equalizer-not-working-properly.html
In your case, Realtek audio console's "speaker fill" function got moved to the sonic studio 3's upmix fuction. But you spent the last month on crazy fixes. It's a fresh install, so nuke your computer and start over again. Install all drivers with the installer exe (setup.exe, asussetup.exe, whatever exe...).