I now had time to test it myself. I got no image when I selected BCM96318REF_P300, but after selecting FAST2704N I got bin/targets/brcm63xx/generic/openwrt-brcm63xx-generic-FAST2704N-squashfs-cfe.bin. I use Debian in Virtualbox. You can get the same setup by following https://openwrt.org/docs/guide-user/additional-software/beginners-build-guide.
Thanks for the input Per. I built it and it worked. I'm a little confused because I thought I had already tried it but I guess I hadn't or had done something wrong.