Fetching Remote Encryption Key for MariaDB
Rather than making it its own server, extend the mariadb service:
mkdir /etc/systemd/system/mariadb.service.d

create a file /etc/systemd/system/mariadb.service.d/prefetch.conf with contents
[Service]
ExecStartPre=/usr/bin/wget -P /etc/mysql https://example.com/my_maria_key.key
ExecStartPost={something to ensure the key is loaded - mysql -e '...' } 
ExecStartPost=rm my_maria_key.key

I don't know if this will deleting the key too early. Consider including journalctl -u mariadb.service -n 50 in your question.