Here is the configuration I am running:
but by default, they are assigned the DNS name servers from my ISP.
This happens because of this line in Your config:
You need to delete it to stop ISP DNS server IPs being dished out to Your DHCP clients
I want all my DHCP clients to get the router as the (forwarding) name server.
I have tried all sorts of configurations to give out my routers IP address instead of the WAN assign address, but nothing works.
You need to specify router IP as "name-server" in Your config, under pool.
set services dhcp pool 192.168.1.0/24 name-server 192.168.1.1
BUT - this is old deprecated way to configure JUNOS DHCP server. Please try the new way - example at https://www.juniper.net/documentation/en_US/junos/topics/topic-map/dhcp-server-security-devices.html#id-example-configuring-the-device-as-a-dhcp-server