How to view an image full screen on an Apple Watch with no UI showing
The native way to do this is to use Xcode 12 and a free AppleID to make a custom app using AVKit and SwiftUI.

https://developer.apple.com/watchos/

Xcode can take your video content and display it as a custom app. QuickTime Player and Xcode can record the screen so you could fake pre-roll Home Screen content and add that to your “actual” video content so you don’t have to tap the screen. You can make a custom icon for your app to mimic whatever you desire. You don’t have to pay for a dev certificate or publish it to the App Store, it will just side load and run on the one watch you use to record your project.
Even though we’re not really a programming site, here’s two links if you want to research this to either hire someone or start down your development path to realize this:

https://developer.apple.com/documentation/watchkit/building_a_watchos_app
https://developer.apple.com/documentation/avkit/videoplayer