Hi @jamescosten
I'll explain using the video control as you mentioned using video controls for creating background effects.
As you may know, you can achieve this by using the "Auto Start" and "OnEnd" properties of the Video Control. First, upload the video files to SharePoint Online (SPO) and then use the AbsoluteURL of the files in the Media property of the Video Control.
To aid understanding, I have added two buttons. Upon completion of video playback, the OnEnd event references the current play mode and press the necessary button from the two available buttons.
Actually, everything can be handled within the OnEnd event.

Media property of the video control is URL(actually get from AbsoluteUri of Attachments field)

OnEnd of Video control

If you want to use a lighter GIF instead of a video file, convenient event, OnEnd is not supported. In such cases, using a Timer to alternately display two different images repeatedly would be a viable solution.
If my explanation is not what you expected, I ask your understanding.
I hope this helps.
Kirkby.