I switched from KDE 3.5 (whenever that was current).

Terrifyingly, I think someone is still maintaining KDE 3.5 proper for OpenSUSE. Then there’s TDE, which is widely available. (But you probably mean 15-20 years ago.)

Not the same distro, but on my system, the relevant file is located at /etc/default/grub. Find the line that says GRUB_CMDLINE_LINUX, uncomment it if necessary, and add your kernel parameter to it (mine has GRUB_CMDLINE_LINUX="acpi_enforce_resources=lax", for historical reasons). Then run grub-mkconfig with appropriate arguments to regenerate your grub configuration.

Per the contents of my /usr/portage/distfiles, the original undivided package is ~500MB, making it the largest single package I’ve got on my system. Splitting it seems like a very good idea . . . but Gentoo generally prefers not to alter upstream tarballs, so I’m likely stuck.

If you’re getting a grub command line, then it’s finding the grub efi payload itself now, but can’t locate the kernel (or possibly the initram or something that’s supposed to be in it). Check your grub.cfg and try to confirm that it’s looking for the correct partition on the correct drive.

One possibility is that it has a degenerate UEFI implementation that will only recognize the efi “payload” if it’s in the fallback location—I had this problem with an older HP laptop. You can force grub-install to place the needed file in this location by passing the --removable switch (you may also need to pass --efi-directory=[dir]), or you can manually copy the file to EFI/BOOT/BOOTX64.efi on your EFI partition if it was already installed elsewhere.