How to show the files in a folder tree?

I don't know about All Files, but you could go into the enclosing folder you want to search, make a smart folder in it, and use kind is document. I think that would find what you're looking for, although I don't know about invisible files, etc. No folders will be shown, although I don't know why that matters.

Perhaps you should try Find Any File; it's a utility designed to flatten folder structures. Whether it works depends on WHY you want to flatten the folder structure; if you just wanna look at the files it probably is more than you need. Path Finder and HoudahSpot can also flatten folder structures and then allow you to do all sorts of manipulations with the results.

In the parent folder type Command+F and set Kind is Document to get a list of all files in the parent folder and subfolders:

Only 19,000 files, hummm... mine has 790,581 files in 176,789 folders.

There is really no need to worry about small or duplicate files as long as you have plenty of free hard disk space.

In fact, you will probable do more harm than good using Gemini or randomly deleting any file's from your Mac.

I see. Sort of. But you still need to be able to ascertain what folder a file is in. I could show all the files in a big folder and gets lots of "readme.txt" files for instance. But they aren't duplicates. I would have no way to know which I could delete without looking at the folder they resided in. Even files with the same size and dates might be essential, as when an application backs something up.

I'd still suggest Find Any File. It allows you to toggle a with and without folders view which might help your quest. Be careful with it though; unlike Spotlight based searches it by default finds all kinds of system files and those are very frequently duplicates, for a reason. You start deleting those and you could crash your machine (badly...like reinstall badly) (I apologize if you are already aware of that risk).

You go to View -> Show Sidebar and may be also Show Pathbar... then you can see the way you wish to see the tree menu.