Jennifer Lewis Oliver
Jennifer is about as close to a one-stop-shop as I was, but she doesn't provide editing services. (She does provide proofreading, but only if the work has already been professionally edited.) She also provides many services I didn't, so if you're looking for additional help with something, ask Jenn.
Amy is a seasoned professional who knows a TON about the business. She does both formatting and uploading, and she may be open to providing other assistance you may need, so go ahead and ask.
Paper & Sage: https://www.paperandsage.com/
Art by Karri: https://www.artbykarri.com/
Wicked Smart Designs: https://www.wickedsmartdesigns.com/
Cover Your Dreams: https://coveryourdreams.net/
Your Personal Graphic Artist: https://bookcoverart.webs.com/
Bespoke Covers: https://bespokebookcovers.com/
Bella Media Management: https://bellamediamanagement.com/
Amy Atwell also has an extensive list of service providers on her site:
I've done teaser graphics for some of you, and if you'd like to try your hand at it yourself, there are several good tools out there, such as Canva, PicMonkey, and Book Brush. You can try these tools for free (at least in a limited capacity), and they have a variety of features, so be sure to compare them to see which one fits your needs.
Note that Book Brush also allows you to create videos and makes it really easy to include 3D covers in your images, so it's my favorite of the bunch.