That’s a second tier question though.
First question: Can the firmware boot from SD at all?
In particular, it must be possible to do this without doing a chain boot from the eMMC, in case the eMMC is hosed. This would be a prerequisite if you want to distrohop.
I don’t think anyone has ever answered this fundamental question. There are multiple topics that discuss this question - without a conclusion, I think.
Second question: Can it be made user-friendly?
This is always going to be relatively low level stuff, so may not be user-friendly.
Do you consider using the BIOS settings or boot disk menu on a regular x86 computer in order to adjust the boot order to be user-friendly?
Both typically involving bashing on some key as the computer is powered on. The boot disk menu tends to be easier to use than changing the boot order in BIOS settings. There is no keyboard on a Librem 5. Would it be acceptable to you if you had to attach a keyboard to change the boot order?
On a spiPhone you have to hold down various buttons during power on, like a Vulcan death grip.