In the old D3lphin file manager, there was another panel called Actions, which had a panel called "Actions" which pretty much just had items in the Actions context menu, always showing on the Side Bar. I always thought that this might be a useful option in modern Dolphin
Personally I'd like this (or a simplified version of it) as well. macOS Finder now has this feature, FWIW.
*** Bug 186185 has been marked as a duplicate of this bug. ***
*** Bug 178528 has been marked as a duplicate of this bug. ***
Adding two links for the macOS finder approach here for reference. - https://appleinsider.com/articles/18/10/05/how-to-add-your-own-quick-actions-to-the-new-macos-mojave-finder - https://support.apple.com/en-za/guide/mac-help/mchl97ff9142/mac
With single click in mind, this might not work that nicely, though. At least it would be harder to open the side bar, since one cannot simply click a file or folder.
Yes, this idea inherently makes more sense for double-click users, as double-click makes selection-without-opening a first-class citizen, and selection-without-opening is necessary for a contextual action UI to work.