(It would be helpful to know what format you are starting with.)
If you cannot jailbreak or don't like KOReader, the path of least resistance is to send PDF of the comic. This will get converted to KFX print replica, but for non color Kindles, it gets delivered as PDF. Print replica does not support 2 page viewing (in landscape), but does support syncing of bookmarks and reading position between the color Kindles and the Kindle apps. But it's the only way to have some semblance of comics added to your Kindle library.
Fixed layout ePub is not supported by Send to Kindle or calibre.
One option is to use Kindle Comic Converter to generate AZW3 for side loading. But you need to start with CBZ or PDF image containers. It cannot take fixed layout ePub. And AZW3 does not have comic features, apart from those which KCC attempts to mimic by slicing up images (if I recall correctly).
In situations where that is the only format, I extract the images and hope for the best: fixed layout ePub can include text which the reading system is expected to render, and which page images do not portray (pages are not required to have an image), so the images may not reflect the intended appearance. KFX output plugin cannot handle them either (it delegates the task to Kindle Previewer, which does not handle fixed layout epub either - it laboriously creates defective MOBI in my experience).
(maybe this works sometimes, I have not tried it:
https://github.com/KeithyIrwin/Fixed...ToCBZConverter.
PyMuPDF is something I've been meaning to try building a converter with, it can input and output various formats, but specific case of fixed layout ePub input isn't referenced in documentation, so it may require more than the few lines of code it gives as an example to pull off.)
I like to leverage comic features of Kindle platform, and the only way to do that is to use Kindle Create & KFX input to generate KFX comic format for side-loading. I also like ToC chapter navigation (which KCC cannot do: you have to edit AZW3 to add it). It's trivial and fast to add this with Kindle Create.
You can also get manga navigation (L or R page order) trivially or with more effort, guided view. Kindle Create lets you preview, crop and resize images add and rearrange delete pages and make sure pages that represent spreads have correct left and right attribute, or you can mark a page to be 'not facing'.
The resultant KPF (a zip archive of the KC project, worth keeping around if you want to want to change something later) can be converted to KFX with KFX output plugin CLI. And then you can generate PDF, fixed layout ePub, or CBZ from it with
from KFX toolbar. PDF and ePub will inherit the table of contents navigation. I like to store all 3 formats in my calibre library, as I am not always gonna target Kindle and other (ePub) platforms pay attention to things like Series metadata transparently and are more welcoming to imported content etc.
It would be super nice if Send To Kindle supported either fixed layout ePub, or better, KPF archive (for comic book or print replica or children's book formats especially). For people actually publishing on KDP, it would let them preview actual device experience before publishing, for the rest of us, better user experience for third party comics.