Extend the functionality of your Librem 5 and do some hardware hacking with the Librem 5 rigid-flex breakout board.
The EXT CON Breakout Board is a rigid-flex PCB which has standard 2.54mm (0.1″) pitched through-holes including signals such as UART1 RX/TX, SPI (3.3V logic), I2C4 (1.8V logic), two GPIOs (1.8V logic for “NFC_EN” and 3.3V logic for “NFC_IRQ”), 1.8V, 3.3V, and GND.
The Rigid section can be fastened down using the center screw that is typically used for the M.2 cover. Once this board is attached it is thin enough to allow you to still place the back cover of the phone over it (so long as you don’t directly solder a header to it). Instead of soldering a header directly to the rigid section, you could instead use a press-fit header such as Amphenol’s 93689-103-03LF (these will require that you cut off the excess length of the end that goes into the through hole, and pinch the remaining end with pliers so it fits well).
The benefit of using this board is that you will not have to modify the standard U-Boot in any way, and you will be able to access both U-Boot and Linux via the RX/TX & GND pins using a standard 3.3V FTDI USB debug cable such as the TTL-232R-RPI.
Read more details about the breakout board on its community wiki page.