Thumbnail job
A thumbnail background is not the whole message. It should support a headline, category, and crop without competing with them.
Reusable system
Build a few repeatable structures: object cluster left, open space right; frame border with quiet center; small desk detail at the bottom. Reuse the structure, then vary color and subject.
Publishing check
Preview the thumbnail at the smallest card size on the site. If the subject or text becomes unclear, simplify the background before publishing.