looking to your services, it seems MariaDB is used on NextCloud only. Before going to kill you system and start from scratch, one last try to setup the database
G_AGP mariadb-server
G_AGA
rm -Rf /{mnt/dietpi_userdata,var/lib,var/log,etc}/mysql
mkdir /mnt/dietpi_userdata/mysql
ln -s /mnt/dietpi_userdata/mysql /var/lib/mysql
apt install mariadb-server
systemctl status mariadb