I’m not sure adding -K for become (sudo) was actually achieving anything because server.yml already runs with become: yes. You may find that you can omit -K in the future. The apt-get connectivity issue may have been just a temporary unavailability, as sometimes happens, and not a sudo issue.
Edit. If you set sshd_permit_root_login: false you’ll need to use -K to be prompted for the become password for the admin_user.
What’s the error?
If this is your repo structure, then you should have repo_subtree_path: ar-advies.nl. (Your CLI output showed repo_subtree_path: site)
If your repo is ar-advies.nl only, without the trellis directory, you should comment out repo_subtree_path.
This thread mentions the same tutorial you mentioned. Perhaps you’ll see some similarities in troubleshooting the resulting setup.