Multi-boot is often used, although normally it would be with an internal partition.
I do recommend that you use the more recent JetPack/SDK Manager, and so you’d want to run Ubuntu 20.04 LTS. If you want to use older releases, then you’d load Ubuntu 18.04 instead.
On the MacBook with the dead dGPU, does it run in any GUI? Does it work with command line? There are limitations to installing and flashing on command line, but it does work. A closer explanation of flash software might help…
Originally there was only command line flash. The Jetson becomes a custom USB device in recovery mode, and for this there is a “driver package” to work with this custom recovery mode USB device. The flash software is somewhat agnostic of what gets flashed, but the second package is the “sample rootfs”. This is pure Ubuntu (in the older releases it is Ubuntu 18.04 or earlier). Then one runs the “sudo apply_binaries.sh” command to overlay the NVIDIA drivers into the “rootfs/” content, which is when it is no longer pure Ubuntu (and becomes known as “Linux for Tegra”, or “L4T”).
During a normal flash the “rootfs/” content almost determines the exact image to flash. The command line arguments to “flash.sh” do result in a copy of target-relevant kernel and device tree and extlinux.conf content to some degree. Then it creates an exact image of the APP (rootfs) partition. “Standard” binary images are flashed to the non-rootfs partitions, and these never change within a given release unless the end user has done some sort of boot stage customization. The generated image (based mostly on “rootfs/”) gets flashed. At this stage there are no “optional” packages installed from a command line flash.
Optional packages, e.g., CUDA and sample programs, only get added (when using the GUI installer, JetPack/SDKM, which runs on top of the driver package as a front end) via ethernet (ssh/scp and commands) of a fully booted system (after flash the system automatically reboots). If you command line flash, and you have the knowledge, you could then use the “apt” mechanism to manually install content. If you had used JetPack/SDKM, then this is done for you (including setting up the rootfs with apply_binaries.sh and downloading optional content).
Technically, if you were willing to work more, then you could install Ubuntu 20.04 on the MacBook Pro (I think this is x86_64/amd64 PC architecture CPU), then you could possibly work in command line mode. However, any of your Macs which have the ability to load Ubuntu 20.04, and have the PC amd64/x86_64 CPU, could do this with the GUI. You’d just need enough hard disk space. They only need an NVIDIA graphics card if they are to install and run CUDA examples on the PC/Mac (people often fail to realize that most of the install step content can be “unchecked”).
I don’t have any Mac experience, so I don’t know what details or roadblocks you’d run into trying to load Ubuntu 20.04, but I know there are many people who have done so.