A good problem to have, right?
The switch between two routers wouldn't be my choice. Even if the ISP router supports link aggregation you are spending more money and not getting full bandwidth on any single device connection.
RB5009 series would be my pick for this. Same price as RB4011, but it has a 2.5Gbps uplink and a 10Gbps SFP+ to connect a switch behind it. You could start out with the RB3011 as a switch and link aggregate if you want.