You may be able to add an IModelTransform to your services to set the thumbnail. We integatrated 3rd party videos and used the thumbnails from them to display in admin:

services.AddTransient<IModelTransform, CustomVideoModelTransform>();

    /// <summary>
    /// Transformer to provider a thumbnail url to the editor area.
    /// This allows us to not store thumbnails content in the repository
    /// </summary>
    public class CustomVideoModelTransform : TransformBase<StructureStoreContentDataModel>
    {
        /// <inheritdoc/>
        public override void TransformInstance(IContent source, StructureStoreContentDataModel target, IModelTransformContext context)
        {
            if (source is  CustomVideoVideoData video)
            {
                if (video.ThumbnailUrl != null && !video.ThumbnailUrl.IsEmpty())
                {
                    target.ThumbnailUrl = video.ThumbnailUrl.OriginalString;
                }
            }
        }
    }

Thanks for the suggestion.

We ended up adding a thumbnail to the media property, then we needed to implement our own IContentCapability, see example

using EPiServer.Cms.Shell.UI.Rest.Capabilities;
using EPiServer.Core;
using EPiServer.ServiceLocation;

namespace Gosso.Plugins.Optimizely.Media;

[ServiceConfiguration(typeof(IContentCapability))]
public class GenerateThumbnailCapability : IContentCapability
{
    private readonly IMyMediaHandlerResolver _myMediaHandlerResolver;

    public string Key => "generateThumbnail";//important

    public int SortOrder => 10;

    public GenerateThumbnailCapability(IMyMediaHandlerResolver myMediaHandlerResolver)
    {
        _myMediaHandlerResolver = myMediaHandlerResolver;
    }

    public bool IsCapable(IContent content)
    {
        if (content is MediaData mediaData and IMyMedia myMedia)
        {
            if (mediaData.Thumbnail is null)
            {
                return false;
            }

            var fileInfo = new FileInfo(myMedia.Name);
            var mediaHandler = _myMediaHandlerResolver.Resolve(fileInfo.Extension); //custom code

            if (mediaHandler is not null)
            {
                return true;
            }
        }
     
        if (content is not ImageData imageData ||
            imageData.BinaryData is null ||
            imageData.ContentLink.IsExternalProvider)
        {
            return false;
        }

        return true;
    }

}