Why buy a Kindle at that point? I have a Likebook Mars that has an SD card slot that I use to load books on to it, which I have never connected to the internet.
In general, I find that Kindle has the best backlight, is fastest, and has a generally very smooth UI. Now, I haven't used the latest version of the OS, so that might have changed, but in general all the non-Kindle E-Ink devices I've used, I've wished it were a Kindle without the spy stuff.
Out of curiosity, do you have ghosting problems with the Mars? You can also connect a kindle to USB and use the internal flash without ever connecting it to the internet.
No more than any other eink? I haven't noticed anything in particular, but I haven't been looking for ghosting.
It works great for my purposes.
(1) Never connect the device to the Internet. There's really no reason to.
(2) Use KFX Output in Calibre, blocking access to the Amazon servers. This is a bit of a hassle but it is doable. Especially since books take hours to read, so you're really only doing the work for a few minutes to get hours of spy-free enjoyment.