Correct, you’d have to switch the Kindle to your Amazon JP account, losing access to all books you might have bought on their “global” Kindle store.
Plus, the Kindle (unless you’ve one of those Scribe devices) would likely be a little too small to read manga comfortably on, needing a lot of zooming in and that’s not exactly smooth on the Kindle.
I don’t read that much manga, but when I do I use a Samsung 11" Android tablet. It’s maybe a tad too large a screen, but on the other hand the 8.2" e-ink Android device I have feels a tad too small for manga (perfect for books though). Perhaps 10" is the sweetspot?
The advantage of e-ink is that it’ll look as close as possible to a paper print, and to be honest I really like how manga looks on it.
The advantage of a Samsung tablet is that you can use the S-pen to scan text and get instant translations via Google Translate’s OCR - these can be shoddy, but are definitely much better than nothing.
The advantage of a laptop is that you could set up Mokuro to get full text extraction and then Yomitan and whatnot. But yeah… I don’t enjoy reading anything on a laptop/computer screen either.
See here for a comparison between Kindle, AIPaper and the S9 tablet.
If you’d like any other details, just give us a shout 
https://community.wanikani.com/t/7-8-e-ink-tablets-with-android-and-pen-support/69434/6?u=cezarl
(ugh, it won’t embed the post directly, sorry)