The Archer C7 isn't really ready for gigabit service, the CPU is not very powerful.
In general when you go to a two box system you want to dumb down the ISP modem / router as much as possible so it becomes a dumb media converter from Ethernet to fiber or DSL or cable or whatever you have. Your OpenWrt box will do your own routing. Then you can set up things like a guest network or VPN server (though VPN server won't work at real high speed due to the CPU).