Ok if it’s only for tranmission we can use a simpler approach.
First remove the stuff we added earlier
# Remove MARK rules
sudo iptables -t mangle -F OUTPUT
# Remove policy routing rules (only the novpn table)
sudo ip rule del fwmark 1 table novpn 2>/dev/null
sudo ip route flush table novpn 2>/dev/null
# Remove the novpn entry from rt_tables
sudo ip route del default via 192.168.2.1 dev eth0 table novpn
sudo sed -i '/200 novpn/d' /etc/iproute2/rt_tables
Also remove the script from the ovpn config, we don’t need it anymore. We will create a permanent iptables rule to route everything send from transmission user through tun0.
sudo iptables -A OUTPUT -m owner --gid-owner debian-transmission ! -o tun0 -j REJECT
sudo netfilter-persistent save
You would also need to deactivate the kill switch in dietpi-vpn to let the other traffic route through eth0.