That's possible with a registry tweak.

Create a DWORD value named:

taskbaritem|mclick

with the value:

5

in this branch:

HKEY_CURRENT_USER\Software\7 Taskbar Tweaker\Mouse Button Control

or use this .reg file:

middle-click-opens-jump-list.reg

What's wrong with setting "hovering" to "list"?

If you want hovering to show thumbnails, but clicking to show a list - I've tried such a variant, it's confusing and makes the taskbar difficult to use.

Meanwhile, this tip can come in handy:

When the Drag within/between groups using right mouse button option is set, scrolling the mouse wheel while holding the right mouse button will show/hide the label of the hovered taskbar button's Application ID.

Have you tried the "Combine when taskbar is full" option of the Taskbar Properties?