Just wanted to add that it felt pretty straightforward buying the digital volumes on Kobo, when I bought the volumes a few months ago.
You'll need to sign up for a Rakuten ID (can't just use your existing English Kobo account, if you already have one), but the registration form is already in English. After you complete the registration, you'll be redirected to the Rakuten web site, so you should manually go back to the Kobo web site. When asked for a Japanese address/phone number, I just looked up the address/phone number of a Japanese hotel.
The advantage of buying on Kobo is it's very easy to remove the DRM, so you can keep a backup of your purchases, or transfer it to any device you want.