Not sure if this will give you what you are wanting, but worth a try.
First, duplicate the video (if you still want to keep the original), then edit the duplicate. In the top right corner there's a gray dot. Tap on that and it looks like it will allow you to use 3rd party apps to edit (not sure). I only had iMovie listed so I selected it. I then clipped the duplicate video and tapped done and it exported the clipped video to my photo library. The first time I tried this, I did get an error, but I tried it again and it has been working fine for me ever since. I uploaded that clipped video to youtube and it only uploaded the clip and not the entire original video. I did notice there is still an option to revert back to original though. It appears it just keeps the original duplicate as a separate hidden file on the phone which will eat up storage, but perhaps the hidden files disappear after 30 or so days. Idk. Guess that might depend on your iCloud settings for Photos.
If you don't have the iMovie app installed, you will likely need to for this to work. Or another compatible app.