Adding
route 10.8.0.0 255.255.255.0 net_gateway
to the "external" VPN client conf on the NAS should make it work. (I'm not entirely sure if it would still be okay if the external VPN also assigns the its clients 10.8.0.0/24 addresses though.
Another approach would be to "masquerade" packets from your own VPN clients to 192.168.1.250 as from the your VPN server, as apparently packets from the NAS to 192.168.1.0/24 are not routed into the tunnel to the external VPN (block-local not in use for redirect-gateway I suppose).
Either of these should avoid the replies for 10.8.0.0/24 from being routed into the tunnel to the external VPN.