Try changing this:

grant all on nextcloud_db.* to 'nextclouduser'@'localhost';

To:

GRANT ALL PRIVILEGES ON nextcloud_db.* TO 'nxtuser@'localhost' IDENTIFIED BY 'nextcloudpassword' WITH GRANT OPTION;

I've been doing similar using SPToolkit. When the user clicks on the login/submit/go button it logs their email address and IP, sends them to a webpage that has training on it and explains what they just did and why it is bad, and finally they have to acknowledge that they read the page. I get a report with all of that within the hour. Works great.

This sounds like a good use case for Rsync. Setup a cron job to run rsync every X amount of minutes.