I managed to install Debian 12 without bumping it up to trixie and then applying your repo and it worked on Debian 12…..
Fixed
In the future, if anyone wants to know how to do this, it is super easy. Download grubfix32.sh from repo.powerprogress.org/debian/install/grubfix32.sh and copy it to a USB flash drive formatted as FAT or HFS. Use the Debian CD to boot into the rescue mode, mount the root partition (for me, it was sda3), execute a shell prompt, and run 'chmod u+x' to make the script executable. Then, run the script './grubfix32.sh' and point it to your boot partition, which, for me, looks like this: './grubfix32.sh /dev/sda2'. Let it do its thing, reboot, and then you can proceed to edit your sources.list and add whatever snapshots you want, as long as they are newer than the image you downloaded to do the install. For example, the DVD image I used was generated in June, so I would not be able to use a repo from December of the previous year or use mintppc source to install his base packages.
My fix above is the only way I’ve been able to install it….hoenstly because of trixie I would if you can create installer images based Debian 12 which will give the user the ability to upgrade and just include a readme stating if you go beyond Debian 12 you may experience broken packages or services….just my two cents…your installer images are rock solid as I used it as well to try it out and worked perfectly..but the more and more Debian generates nightly package builds the more I have a feeling are going to broken. With this method it allows the user to be on a snapshot so your less likely to have something broken and also gives the user a choice between bleeding edge and having something that’s a little more stable…