I don't know of any way to avoid having Amazon deliver books in KFX format on e-ink Kindle devices that run firmware versions greater than 5.6.1.1.
The latest version of Kindle for PC (or Mac) also supports KFX, but it can be forced to revert to using the older kindle formats using Method 2 documented
here. If you do that then most books will be delivered in azw3 format and some will have an additional azw6 file with higher resolution graphics.
Note that the file extensions will not be as you would expect from an e-ink Kindle. The azw3 files will have the extension .azw and azw6 files will have the extension .azw.res on the latest version of Kindle for PC/Mac with KFX disabled.