How to upload AWZ3/KFX to Amazon Cloud? (more details why inside)

Hi everybody,

I am using a Kindle Oasis and the Kindle App on my iPhone and iPad – therefore, I upload my (own non-DRM) ebooks with "Send to Kindle" to the Amazon Cloud and from there to all three devices, so that the reading position is kept synched this way (treating my ebooks as items in the category "documents").

However, I'd also like to use the bold-controlling feature on my "real" Kindle as my favourite font "Bookerly" is a bit too "pale" in standard setting. And here comes the problem: This feature does not work with MOBI, only with AZW3/KFX files – which I am not able to upload with "Send to Kindle" or email. Of course, I could sideload a AZW3/KFX version to my Oasis with Calibre and USB and upload the MOBI versions to iPhone and iPad – but in this case, the devices won't kept synched.

I just want to be sure: Do I miss some other upload way Amazon offers which accepts the AZW3 format or do I really have to live with this? I fear the last after my already done researches, but I dimply don't really understand why the upload feature still is limited to MOBI ... If there's a solution for this, I would be grateful if somebody could give me a hint.

Thanks in advance, happy new year!