P: Drag Stacks from Collection to Folders

LrC's normal safety-first approach means it will not affect or include currently hidden stuff along with what is visible. This also works when a folder or collection view's full contents are filtered down to a smaller number of images showing. You can think of a stack maybe, as a kind of filtering.

Based on that, one answer may be to employ image attributes which will span all contexts, instead of relying on context specific stacking. The stacks you have can be expanded / collapsed so some attribute can be bulk applied differentially, and then the stacks all expanded and the whole lot dragged into Collection. I cannot check just now, but believe the Added sort order may / should respect the same image sequence you see for the folder view with all expanded stacks.

But now you can view-filter this Collection on the basis of that attribute for the same effective outcome as expanding / collapsing all stacks in the folder context: and you can now view-filter the folder too, in the exact same way. If that commonality is important to you, it might lend support to using rating / whatever rather than stacking for future such tasks.By filtering the view where you are applying this attribute whatever it is; each marked-down image can then automatically get hidden from view by the filter straight away, as you go. Similar to stacking it away; albeit without that same subgrouping going on.