Too many Alt-Tab repetition.. [which is moreover too slow]

My humble desire is to very easily bring up a list of running applications (~tasks, ~windows), and very easily select one.
(Because my screen is not huge, windows are normally stacked. And I do not use "always-visible" task panel because it always reappears (when mouse is near the bottom) even when I set it "can be overlapped".)

I mean: I hit one key (not modification key + another), the window list appears, and remains at top until I select one among them or press Esc. Selecting could be achieved with the mouse, or via keyboard (shortcuts, underlined letters).

You can try to set the alt-tab combo to:

1. System-settings->Keyboard and mouse->Global keyboard shortcuts

Select kwin. Find the "Walk through windows" and set the keyboard shortcut to whatever you want.

2. System-settings->Window behaviour

Check "Traverse windows on all desktops"

3. System-Settings->Desktop

In "Effect for window switching", select "Present windows"

The only thing missing, as far as I can tell, from your description is that you need to keep the shortcut key pressed - which is why a modification key is needed. The default, in this configuration, is that alt-tab brings up the opened windows of all desktops (good for an overview). Keep alt pressed and tab switches between the windows.

An alternative (or combination) is to use

System-Settings->Desktop->Screen edges to set up showing "Present windows (for all desktops)" to a screen-corner. Move the pointer to the designated corner and you get exactly what you ask for (I hope).

Another alternative is to set:

System-settings->Keyboard and mouse->Global keyboard shortcuts

Select kwin. Find the "Toggle present windows (all desktops)" and set the keyboard shortcut to whatever you want.

I hope one of these suggestions will do what you want.