Yes, you can put the Brume 2 in front of the main router, since your ISP connection is not over coaxial cable like mine. For your fiber Internet connection, you have to fiddle with the VLAN configuration to work with the ONT, according to your ISP.

After you get connected to the Internet, you can try redeploying the main router as a wifi access point, but it all depends on its capabilities.

I do not know Portmaster + SPN and whether it is available on OpenWRT.