MacBookPro3,1 15.4" (Mid 2007): unable to boot from a live USB

Hi,

Has anyone ever managed to boot from a live USB on a MacBookPro3,1 15.4" (Mid 2007)?

I've just spent my week-end trying various things, but never got very far. I'm new at this though, so I was hoping the community would point me out to things I haven't tried yet.

What I have tried:

• I've been trying with Ubuntu 18.04 Desktop AMD64, but I'm open to other versions and flavors. I just want a version that will remain supported for a few more years. In contrast, Mac OS X El Capitan (the last macOS version supported by this MBP) will reach end-of-life support this Fall — hence the motivation to migrate to Linux.
• The first thing I did is install rEFIind, following the instructions here. This seems to be working, even though I realized later that this probably wasn't even needed to boot from USB (the Mac boot loader is sufficient).
• I then went on to create a bootable USB using UNetbootin, following the instructions in the previous link. That didn't work, in that the USB would not show up in any boot loader (neither rEFInd nor when starting the Mac while holding the ALT key). I think I have identified that this was due to the fact that the instructions mentioned using the GUID Partition Map, while I should have used MBR.
• Once I had an MBR live USB created using UNetbootin, I got all the way to GRUB 2, but when selecting the first option (to boot from the live USB), all I got was a black screen. Following the previous instructions, I tried with the "nomodeset" and "nvidia.modeset=0" options, but that didn't help.
• When I try to check the disk, I still get a black screen, and after a while I get the following errors: "Couldn't get size: 0x800...0e" and "MODSIGN: Couldn't get UEFI db list". A search on the web suggests that this may be an unrelated, non-fatal error.
• I then tried various other ways to produce the live USB. For instance, I tried without UNetbootin, using dd instead. I also tried Etcher.
• At some point I got an error message "error: file `/boot/' not found". A search on the web brought up a thread about ISO-2-USB EFI-Booter for Mac, so I gave it a try as well. It took several long minutes to complete the "Loading RAMdisk" step, and then it got stuck for hours at the "Fasten your seat belts" step.
• I've also tried to burn a DVD instead of USB, but my "Super Drive" is not being very "super" anymore, i.e. I can't get it to burn anything other than my time.

Suggestions on what to try next would be very welcome. Thanks a lot in advance!