> One downside to storing these images on the (FAT) EFI System Partition is it is not possible to make it part of a software RAID device, so if the device the ESP is on dies suddenly, the boot fails

It is possible to make ESP part of MDRAID array of level 1 with metadata v1. True, the boot starts with single disk and only later Linux activates the array. So if your boot fails before that happens, reboot and try the other disk.

Or alternatively, don't set up MDRAID array for ESP, just have the ESP mirrored on all disks.