So what works for me for reflashing is to do it the same way we do on desktop PCs. i.e. instead of the Purism flash script, I figured out from the script what the URL to the .img is, download that image of PureOS for L5, and then write it to an SD card. Then I reboot while holding Volume Down causing the L5 to boot the SD card. Then on the card, I use the standard Linux dd command included in basically all distros to directly read or write from a file or device, pointed at the eMMC of the L5 while it is not in use. [dd is also what I use for writing the img to the SD card]
So to flash we can dd with input file as the img and output file as the L5’s main eMMC storage. Or we can do the process in reverse to make a full backup of everything on the L5 including the system.
By buying an SD card larger than my L5 main memory, I recently had an experience where dd in this manner let me basically do a perfect transition from old L5 device to new L5 device. Unlike the flashing script that requires a computer and operates in one direction, dd is simple and straightforward and can seemingly do anything.
It can probably also break anything, though. Be sure to make backups.
Edit:
And whether reflashing from a PC or from an SD card, the most hardware you have to touch is the SD card tray. No disassembly required. But some people say the SD card tray shorts and burns cards unless you have the device off while opening and closing the tray. Better safe than sorry on that one, in my opinion.
Edit 2:
Also I didn’t check if this is true, but I think booting from an SD card might boot eMMC main storage first - to a limited degree - then detect user wants SD card. So you probably should never zero out or nuke your eMMC while planning to rely solely on SD card boot because for all I know maybe then it wouldn’t boot. But it’s still a convenient thing to have SD card booting.