Historical Religion Series Cover Design

I have fifteen already-published KDP titles that currently sit under a plain template. They trace the story of world religions from the earliest BCE records right up to contemporary practice, and I want the covers to reflect that rich historical sweep while still feeling like a unified series... (Budget: $30 - $250 USD, Jobs: Adobe Creative Suite, Adobe Illustrator, Adobe Photoshop, Book Cover Design, Graphic Design, Illustration, Illustrator, Photoshop)