I don’t know that RAID 0 is that big a deal when the next generation design can just get a faster SD card / reader, and, as you commented already, there isn’t any info about what the spec of the SD card reader is currently for the Librem 5.
One concern about SD cards is how reliable they are. It is all very well for them to be high capacity and fast but if they are not reliable, I won’t want it. So it needs to be a quality card. By suggesting RAID 0, you are making this problem worse.
An alternative approach, for a future version of the phone, would be to keep the existing eMMC drive but allow you to swap the roles of the two drives i.e. boot from SD card and use the eMMC drive as additional storage. Whether that fully makes sense depends in part on what capacity SD card you plan to use. That only addresses one of your bullet points however.
It is possible that this could partly be achieved by using a chain boot, which may avoid the need for any code changes to the “BIOS”. (This is the reverse scenario to the Raspberry Pi, where it wants to boot from SD card by default but some people may attach additional storage and really want to boot from that.)