Thank you for the welcome. I am not yet in the camp that "if you cant measure it, it doesn't exist".
Wildings
Whatever camp you happen to be in, welcome to the Ah !
I am in the live and let live camp most of the time : whatever you enjoy: go for it.
If you've been listening and playing with audio for 45 years, you certainly have earned your opinions and positions.
Is everything measurable? Can you measure everything you hear in an audio system? Excellent philosophical questions. Considering the items you mentioned (soundstage, instrument placement, depth etc) I would say there's room for discussion simply because none of those items exist outside of your brain.
Your soundsystem, headphones or speakers, vibrates the air with some waves in the audible spectrum. That's about all it does. The music, the soundstage and all that goes in to the stuff we enjoy so much is assembled in your fantastic brain and presented to your conscious mind as a song. There is no soundstage or stereo image at the front of the room. Only inside your brain.
So, I would suggest there's plenty to be experienced with an audio system that is not strictly measurable with a tool. I would also say most of the important elements can be measured in one form or another. But everything? I think there's room for a little wiggling.