Most of the time,
I just use a frame from inside the video I'm uploading.
I'm also using TubeBuddy to help me edit it a bit easier.
To each thumbnail I'm adding my logo, that way all of my videos thumbnail looks a bit distinctive from the rest.
Sometimes I add a logo of the topic (like the Husqvarna logo when I did my new bike reveal, or a cigarette photo when I quit smoking)
I also sometime add some text with my distinctive font and color scheme.
I think that in thumbnail design terms, the "next level" (for me at least)
will be to outline something in particular.
View attachment 3849