I have arrived at a similar conclusion using plans and sections at appropriate scales for live detailing. My use of 3D documents is still limited as I find the process of arriving at a final view to be "troublesome" for various illogical reasons e.g. my latest problem was a 3D document only displaying part of what I wanted to show and I could not identify why the illustration was cut short, the other three similar details all worked as planned. In the end I had to recreate the view.
I appreciate why the detail tool works as it does, but as you say it is "very old fashioned". What GS should be doing is simply modifying the detail tool to work the same as the section tool and provide an option for "auto update" to keep the view live. This way you can cover all bases and have live, static (with possible overlays) or independent for free editing.
I think the 3D details you propose are a valid solution, I just don't think AC is quite up to speed yet when it comes to pulling the required illustration information out of the model. I also believe GS need to look at element scale sensitive visibility e.g. a LOD option that works for details down to 1:1, but that a different issue.
Apple iMac Intel i9 / macOS Sonoma / AC27UKI (most recent builds.. if they work)