I use bookwalker and amazon kindle, except I’ve moved over mostly to bookwalker because I have a gigantic amazon US kindle account and you can’t really swap between the JP and the US account that easily on any device.** I do miss the great JP-EN dictionary that Kindle has, though. My suggestion for beginner reading is Kiki’s Delivery Service here: https://bookwalker.jp/series/11646/list/ Just add to cart like you would with the US Bookwalker account and check out - you can buy off your US bookwalker account so the checkout process is the same. BTW, bookwalker has tons of great sales all the time so you might want to wait until they have those 100 yen sales, or win one of the 5000 and 10000 yen monthly giveaways that the US bookwalker does (I won 10000 yen once), since they work for both the US and JP store.
**This is not strictly true - you can buy ebooks off other Amazon country accounts in a single account (think JP is included but not 100% sure - I know you can with UK Amazon) with a separate mailing address within each country but if you have an Amazon Prime account, they’ll kick you out of prime every time you change out countries. If you don’t have one or aren’t part of a Prime family account, go wild! Set up separate addresses.