I’ve been working through this on my old 2012 Mac Pro. My issue has been my graphics card. In that era any non-mac graphics card won’t give you the boot-screen you need to choose which OS to use, or even to choose a USB boot for installing the Linux os. I got Refind (a super light bootloader) to work with a bit of extra tooling. You can also use opencore, but that is more challenging and makes a lot more changes.