I can't currently add this as a comment to Spades solution so it's posted as another answer.
Anyways, I was seeking a pure Ubuntu implementation as I only use Ubuntu. I started with Spades procedure. Found basically three things of note.
1) What is important is the GPT partition table for the flash drive. You can change the partition table of a flash drive by using the GParted program.
You can install GParted by doing:
sudo apt-get install gparted
Then you can just use the Startup Disk Creator that comes with Ubuntu instead of some other program.
2) On the note of using the 32-bit EFI, I added the file into the ISO before creating the start up disk. On Ubuntu, /EFI/boot is read-only once the flash drive is created
3) I put rEFInd on another flash drive so the Mac Mini had two flash drives inserted to boot into Ubuntu.
Everything else is basically the same procedure. I've written my steps in more details on my blog here: https://jeffreywong.ca/tutorials/how-to-install-ubuntu-18-04-and-variants-on-a-2007-mac-mini/