Hello, Frédéric From France.
There are many questions here.
1) fault tolerance.
2) recovery after exiting the working state of the root file system.
(1) CD card, eMMC, are all flash drives.
Technologically, they are different, but they all have a limited resource for recording.
Armbian tries to reduce the number of write transactions using the overlay file system.
In fact, system write transactions occur in a file located in RAM and are synchronized
with the real file system on the drive at longer intervals than the system ones.
(2) You want to place the entire root file system on an LVM partition whose type is linear.
In this case, you have to recreate the initrd to make it work.
And if your drive at the physical level starts to fail or the LVM partition is full, you get very big difficulties for data recovery.
packages/bsp/common/usr/sbin/armbian-install
This is a very simple script that makes a copy of the OS on your CD card from which you booted and work on an empty nand, SATA, eMMC or NVME.
Your drives are eMMC and NVME.
What would I do at the very beginning of the journey, when both drives are clean?
Boot from the SD card and install the system on eMMC.
Boot from eMMC. (for health check)
Mark up the NVME drive as LVM or as BTRFS. (The BTRFS file system will provide you with all the necessary functionality.)
Make logical partitions for target folders "/opt" "/var" "/srv" "/home".
Boot from the SD card and copy the contents of each target folder to the corresponding logical partition while retaining all rights for files.
Add the necessary mount entries in the /etc/fstab file located on the eMMC.
Boot from eMMC. You are the master of the situation!