Add a new routing table, needs to be done once.
echo '10 isp' >> /etc/iproute2/rt_tables
You need to masquerade the traffic from the wg to the vpn tunnel.
iptables -t nat -A POSTROUTING -s 10.6.0.0/24 -o tun0 -m comment --comment wireguard-nat-rule-2 -j MASQUERADE`
Then you need to make a rule for the wg-server traffic and send that traffic to the new routing table
ip route add to default via 192.168.10.1 table isp
ip rule add iif lo sport 51820 to default lookup isp prio 15000
Change 51820 with the wireguard source port you are using. These commands should be part of the up-script for the vpn-client connection.