If you really want to do this, be aware that the sysupgrade.openwrt.org server exposes all of the request contents, so be sure you are ok sending it over the internet and storing it on a public server.
Image builder works great. After unarchiving a backup, changing the settings and including the directory in the image builder, I got a working image. After flashing I could access the router again. I had to flash twice because luci is not included as a default package and I like the web interface to check things. So I included the luci and ca certificates packages in the next image i did build. I also had to add list dns '192.168.10.1' to the lan interface in the /etc/config/network file to get dns resolving working.
It is very handy for routers that do not use the default IP address 192.168.1.1
BTW I turned ipv6 resolving off on my dns server, because my isp does not support ipv6 and else openwrt.org could not be reached by the router, an Zyxel EX5601-T0-ubootmod.