I may not be an expert, but I do have a degree in multimedia/design. I think it's very important for author branding to keep things similar. In an episodic story/serial some authors even have the exact same image on their covers and just change the title. For a full-length book series, I am doing this for my second book:
- Keep same font, size and placement for title and author name
- Change main image, but with same model/character (or recognizable object/building) in a different pose with different background
- Change color scheme - color of text matching color palette of images or white/black to stand out
- If you have quotes/endorsements/subtitles to put on the cover then make sure they have the same font, size and placement as well