How do I display previews for all the files in a directory including the ones inside subfolders?

You can switch to the Details view mode, where there is an option in Dolphin's settings to enable Expandable folders (Configure Dolphin -> View Modes -> Details). With this you can expand the folders as you desire, and then use the Preview button to toggle the previews.

Thanks for the reply
expandable folders was already checked, I can see all the thumbnails just not at the same time
I seem to remember on an older version being able to search for * and having the result be everything at once