There are very few times I suggest making your eBooks exclusive to Amazon, so what's the easiest way to handle those other platforms?
I use and recommend Draft2Digital.com for all of your non-Amazon publishing and distribution needs.
eBook files are not all alike. Each platform has its unique requirements, so a file that's optimized for Kindle won't be optimized for iBooks. An iBooks file won't be optimized for Kobo, and so forth.
All you need to publish your eBook to multiple platforms is a properly formatted Word document.
From there, you can publish and distribute via KDP and all of the other platforms in two simple steps: Upload the file to KDP, and upload the file to Draft2Digital.
Each of these publishing platforms will turn a properly formatted Word document into the perfect eBook file for you.