i have the Kindle basic (8th generation) and i find it just as easy to books on it using Calibre as i do putting books on my Kobo's. i prefer reading kepub to epub so i use to use Calibre to do that although now i use Kepubify instead, that said i still do use Calibre for tidying epub's up and converting to the Kindle formats.
i prefer the software on the Kobo albeit the older firmware versions therefore i use my orignal Aura 6" (all) of the time, my Kindle basic is sat motionless inside it's cover.
of course it's each to their own
best wishes koboy