After installing, you get a 2nd blue gear (for options) in the player UI (top right).
You can configure it to hide all UI elements "hide various text and buttons". This way the seek bar won't be shown when pausing the video. You need to reload the page before it applies.
After that you can hold Ctrl or Shift to show the hidden UI (or use keyboard shortcuts).
Sadly I don't know how to hide the X-ray panel in the top right (but it goes away after a few seconds). It even moves the subtitles up and makes them transparent when X-ray is visible. Hopefully the extension will be updated to hide X-ray too.

works like a charm

ありがとうございます。イライラが解消されました！