Detect progress prefab instantiation

I encounter a problem when I want change of level in my game. I don’t use multiple scene cause it’s not necessary.
My problem is I can’t detect if the progress of all “Start” method of each prefab is over or not and due to this I can’t made a “true” progress bar loading.

There is a way to do this ?
The only thing I see is to put every prefab to instantiate in a list and instantiate each element one after the other. When I reach my list count I can except it’s over. But there is another way to create a “true” progress bar between 2 levels without use different scene and tho the “async.progress” value ?

Thanks in advance.

No, without async there is no “progress” as such; everything happens in one frame.