Some graphic novels don't work on ANY real Kindle. Only Apps (and not all platforms) and Fire (really an Android tablet).
If your Kindle FW is at least 3.4.3 then it does KF8/asw3 as well as old mobi.
If Amazon on the download is sometimes giving azw3 files and Calibre says they are KF8, then there is no point in a newer Kindle.
FW 5.x.x and later I think does KFX, but you only get that to an App or on WiFi/Mobile direct to a Kindle, never on "Download to PC for transfer via USB"
Certainly 5.12.2.2 does KFX, and I think I have a PW3 with FW 5.12.4 and one with 5.13.3, and there is no difference on what is downloaded. Both get KFX direct and azw3 via PC download unless a really old title.
Also the kindle you have doesn't even need to work!
Official Amazon FW page for Kindles.
https://www.amazon.com/gp/help/custo...MQC26VQQMM8XSW