Extends Unity VideoPlayer with methods to display the videos first frame, with with either a yielding coroutine or callback function when the video is ready to be displayed.

VideoPlayerExtentions.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Video;

public static class VideoPlayerExtentions 
{
    public static void ShowFirstFrame(this VideoPlayer player, System.Action onCompleteCallback)
    {        
        VideoPlayer.FrameReadyEventHandler frameReadyHandler = null;
        bool oldSendFrameReadyEvents = player.sendFrameReadyEvents;
        frameReadyHandler =  (source,index)=>{
            player.sendFrameReadyEvents = oldSendFrameReadyEvents;
            player.frameReady -= frameReadyHandler;
            onCompleteCallback();            
        };

        player.frameReady += frameReadyHandler;
        player.sendFrameReadyEvents = true;

        player.Prepare();
        player.StepForward();        
    }  

    public static IEnumerator ShowFirstFrame(this VideoPlayer player)
    {
        VideoPlayer.FrameReadyEventHandler frameReadyHandler = null;
        bool frameReady = false;
        bool oldSendFrameReadyEvents = player.sendFrameReadyEvents;
        
        frameReadyHandler =  (source,index)=>{
            frameReady = true;
            player.frameReady -= frameReadyHandler;
            player.sendFrameReadyEvents = oldSendFrameReadyEvents;
        };

        player.frameReady += frameReadyHandler;
        player.sendFrameReadyEvents = true;

        player.Prepare();
        player.StepForward();    

        while(!frameReady){
            yield return null;
        }    
    }  
}

Ah okay I understand the logic now behind how it works.

I ended up coding something very similar to your suggestions with saving the initial frame, although I'm gonna experiment with AssetDatabase.ImportAsset(path) now, I hadn't seen that earlier.