As far as I can see, the shutdown of Mincomi doesn't affect the volumes at ebookjapan site.
They are still there, and it is possible to buy them. Unless the 6th volume won't be released at ebookjapan (which we'll know in 2-3 months or so, when there will be enough chapters released to fill the volume), there is no reason to panic.
The shutdown of Mincomi of course affects people who want to be able to read the latest chapter in Japanese for free. These people should be able to read using these iOS and Android apps linked in
this tweet. Note that this has to be the application in the linked tweet. There is another ebookjapan reader application in Google Play that
doesn't allow you to read the latest chapter for free.
...and with that said, if you like this manga, please consider supporting the mangaka by buying his work. The registration at ebookjapan is quite straightforward, the site accepts credit card payments, and Google Translate works well with the text used on the site itself. Unlisting from ebookjapan's mailing list isn't that straightforward though, but it's possible.