I have the same model Mac as you. The machine is a 2.4 GHz 2007 20-inch iMac. I downloaded to my 2011 iMac the English version of the current 32 bit Windows 7 from Microsoft's website Download Windows 7 Disc Images (ISO Files). The file name is
Win7_Pro_SP1_English_x32.iso. The 2011 iMac is running High Sierra (macOS 10.13.6). I used the 2011 iMac to burned the ISO file to a DVD‑RW.
At startup of the 2007 iMac, I held down the option key until the Mac Startup Manager icons appeared. I selected the text
Windows under the DVD icon to boot from DVD. I was able to boot from the DVD without any problems. I would assume if you have followed the same procedure, then either you did not successfully burn the DVD or the optical drive on the 2007 Mac is faulty. Since you have tried using the Boot Camp Assistant, I assume the Mac can run OS X without any issues. Also, you stated you verified the firmware is up to date.
There a some fairly old posts here a Ask Different regarding the installation of 64 bit Windows 7 on a Mac where the optical either no longer works or has been removed. For example, see the question Boot camp install of Windows 7 issue, no bootable devices.
More recently, a post has been made where Windows can be installed without using a DVD or a virtual machine by first installing Debian Linux. Once Debian is installed, Grub can be used to boot the Windows installer. See this answer. In your case, this answer would have to be modified to use a 64 bit Debian Linux and Windows 7.