The R4S shapes Gbps with CAKE SQM/QoS no problem.
Have you done any testing with your R5S to see what it can actually shape with CAKE before it runs out of CPU? I would guess the slower CPU in the R5C and R5S runs out of CPU to handle CAKE somewhere around 700 Mbps.
The R5S does look interesting (I'd probably get the R5C to get the USB ports), and a PR was just started to add OpenWrt R5C and R5S support in SNAPSHOT (hurray!), but the R4S seems like a better option for up to 1 Gbps routing.