Hi,
it seem to be correct.
I do all the work only with ZS, I also have 2 ISP both dinamic and I use the Loadbalance and seem to works OK the only thing that I also do is the firewall and DHCP all inside ZS.
I tested also a configuration like you suggesting ( having the DHCP and Firewall on another hardware after the ZS HW ) but it add some latency to the traffic so I dismiss the second firewall and setup all in ZS.
Cya