If you are the 'drag and drop files' kind of person the Koboes are definitely the way to go, assuming you REALLY aren't interested in extra functionality, in which case it would always be better to get an Android e-reader as salamanderjuice mentioned to get more flexibility in the future.
Note that you can download software to Kobos made by users here that IMO make for a better manga experience. KOReader and Plato, see below:
https://www.mobileread.com/forums/sh...d.php?t=314220
But you don't have to worry about any of that or even about Calibre for that matter because the Kobo will work for manga and just sliding files in there straight out of the box. My advice: remove all non standard qwerty characters (for example Polish-specific letters) from the file names as I've had to reinstall my device multiple times because of French accents. I wish I were kidding.
Whatever you do, do not get a device with a screen smaller than 7.8" or so. It will just be unpleasant for manga.
Alternatively you could go for the Pocketbook Inkpad line if you prefer their placement of physical buttons. They have a color model that would be good if you ever want to read color comics, but be aware that the colors will look like old newspaper and that it is preferable to buy a black and white e-ink screen if you ONLY read black and white manga as the color layer makes the greys and overall resolution worse off.
If you want to support your local industry, you can also try the Inkbooks which are made by a Polish company. Their main selling point is that the physical turn page buttons feel excellent, more so than any other device I have tried, but I would not otherwise recommend them unless you can take advantage of their Polish-language bookshop etc. As I don't speak it I cannot tell you more about that.