I want to have an installer with custom boot parameters. This is so as not to have to enter them manually each time, for I want to use the installer for multiple installs.
Assuming a CD/DVD installer, you have to remaster the installer image with modified kernel parameters in its boot loader configuration. For BIOS (not UEFI) boot on a stretch installer ISO the default boot option is described in
isolinux/gtk.cfg. Change the
append line to your liking, then build a new ISO as described at https://wiki.debian.org/DebianInstaller/Modify/CD.