I made this:

It's called "Clip Timeline". (It's a commercial product - 8 bucks).
It sits in the viewer... but can be added to an "adjustment layer" and can be turned on and off easily.  The first thumbnail is the first frame and the last thumbnail is the last frame. The 9 other frames are spread out by 1/10th the length of clip that the effect is applied to (you can calculate how the thumbnails will appear.)
It uses a copy of the current state (processed effects) of the clips in the timeline.
Playhead position is indicated roughly by the over/under bar that animates (currently on the 4th thumbnail here), and an optional playhead indicator.
Thumbnails automatically update depending the time length (if you're using an adjustment layer title) and whether it is moved.
There's also a technique you can use that if the main subject is "off-center"/beyond an edge, the pan can be adjusted including all separate thumbnails individually (if that makes sense...) [positions can be keyframed over time!]
It works with any display aspect ratio clip (best applied to an adjustment layer using 9:16 clips in an HD DAR project).
The timeline can be placed at any vertical position of the screen. A contrast control can make the thumbnails brighter or darker than the "background" (easier to see OR a lower third background?)
If you'd be interested, I'll try to get you a link.