current latest download of LinuxCNC V2.9.8 will not install GRUB on several PC's

But the default Debian download on their home page is the netinst ISO, same as LinuxCNC. You need to follow the link to other downloads and select the complete installation image to install on an offline PC. The Linuxcnc 2.8 installer on Debian 10 used the same installer so nothing has changed for many years.

I found out the hard way this week that the Debian installer deletes packages that are not used at run time. eg if you go to the trouble of deploying LinuxCNC built from source in the installer, (eg a 2.10pre version), you must reinstall the dependencies if you want to rebuild the image.  The build time dependendcies are removed before the ISO is created.There are settings that are meant to prevent this but they did not work for me.

The days of Debian fitting on a single DVD must be over. I thought they maxed out at 4.7 gb. The image I built  yesterday came in at 5.15 gb.

I'm off to buy a CPU cooler today. Debian 12/13 do not manage heat well or my 20 core Lenovo i7 is underspec in the CPU cooling department as it regularly overheats and hangs during video rendering or compiling. Hopefully I can shoehorn one in in this small desktop case!
 

Also be aware that Trixie no longer uses /etc/apt/sources.list.d
Instead it uses the new deb822 format and places its sources in /etc/apt/sources.list.d
And as you said, for a DVD install, the source files will be configured to look on the ISO, not at the Debian mirrors