Many authors who want to self-publish think they must choose between Amazon KDP or Ingramspark; you can choose both for different exposure. #selfpublishing
Additional thoughts from https://sleepingcatbooks.com below on how to approach Ebooks and Print books.
Ebook: If you don't use Amazon KDP Select (which is exclusive), you can use all these platforms (KDP, B&N, Kobo, Apple Books, Google Play) and use Draft2Digital to reach the rest. Do not use IngramSpark for an ebook. By going direct where you can, you'll have higher revenues, more control, and access to any promotional opportunities the individual platforms may offer.
Print: You can use Amazon KDP and IngramSpark together, using the same ISBN (that you own) for the same edition through both. Set up the project on both platforms by entering the ISBN, then save that project as a draft on both platforms. Do not enable KDP Expanded Distribution. Do not set a future launch date on KDP. Once that ISBN is in both systems, you can proceed with publishing in whichever order you like without either claiming the ISBN is already in use.
NOTE: If you don't want to format your book, see the Self-Publishing Services Rated post for a link to self-publishing services from the Alliance of Independent Authors rated by the following categories: Excellent, Recommended, Mixed, Watchdog Advisory, and Caution. Also, be aware that many scam publishing companies are pretending to be affiliated with Amazon by using Amazon in their name. When in doubt, check them out using the Self-Publishing Services Rating list.