I was just pointing out to OP that Kobo doesn't accept Kindle .AZW ebooks. Hope you don't mind!
I merely gave a solution to the potential format problem you raised, assuming the OP can overcome the DRM issue he was aware of.
the closed Amazon ecosystem.It isn't closed! Kindle will accept ebooks from outside Amazon and they may be in non-Amazon formats like .MOBI.
The closure I was thinking of was of the content, not the reader platform. As Amazon do not permit third parties to use the Kindle DRM system, there is no way to use Kindle content supplied with DRM on non-Kindle devices or apps other than by circumventing the DRM.
I'd rather have DRM free content, as I believe DRM impedes the rights of a content purchaser to use their content whilst doing nothing to prevent piracy. If you're going to breach the copyright in the content, there's no further legal peril in circumventing the DRM.
If there is no alternative to DRM, but there is a choice of formats, I'd rather buy content using Adobe DRM, which works on a variety of devices and apps.