SOLVED:
For anyone who wants to turn a video block or gallery into a full bleed image/thumbnail before the video is played, I have figured out a workaround without CSS code unless you want to change the play button like me. I used CSS to make a larger play button which is easy to google.
This full-bleed video solution I have come up with is WAY more simple than all the code injection options I've seen.
Simply create a video block or gallery on a fresh page using an Index, so that you can set a background banner image on your page.
Set your desired video thumbnail image that you want to be full screen/ full-bleed as the background banner image.
Then add your video block/gallery on top of you banner image.
Obviously your video block now blocks part of the banner image BUT if you upload a custom BLANK transparent .png that has NO IMAGE on it as your video thumbnail, your video block will now become transparent except for the play button!
Now you have what looks like a full bleed video that scales with your browser, and plays at the correct resolution with sound!
See example here:
https://www.goldenllamaproductions.com/multicare-brand-evolution