Hey there, I just launched an app for Nextcloud for authenticating users via ISPConfig Mail User API. https://apps.nextcloud.com/apps/user_ispconfig It has some options regarding quota, group membership or username mapping. See the readme for details: https://spicyhub.de/spicy-web/nextcloud-user-ispconfig/blob/master/user_ispconfig/README.md
Just released update 0.2.2 that allows you to set preferences for new users for other apps. e.g. set ojsxc preferences for the xmpp chat app.
