Actually the correct way to do this is: just compare the two firmware and find out the differences, when they are running.
You should check the old firmware’s overlay to understand what is your modifications after firmware boot
Generally you should check more details on /overlay/upper/etc/config
You should focus on checking the details of each file to see if it is necessary to have them in your firmware.
Generally you should focus on
/etc/config/network: network, vlan, wan, lan, etc.
/etc/config/wireless: wifi ssid and key
/etc/config/firewall: ports, nat rules
/etc/config/dhcp: dhch and dns
/etc/config/glconfig: gl specific settings, e.g. login password and cloud
/etc/shadow: root password
For others, they are just for specific applications. You need to check each application to see if you want to hardcore settings.
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