Load Balance is automatically triage and selected the uplink (WAN) interface and cannot define which interface takes precedence, it is not Failover.
Your requirement is to specify which clients go to which uplink (WAN) interface, which belongs to policy routing. The GL firmware does not support it at the moment.
You can learn about the third-party plug-in mwan3
https://openwrt.org/docs/guide-user/network/wan/multiwan/mwan3