I can confirm that if you do a fresh install you can set up the folders again and have access to all data, but you need to do things in a specific order and with the array disks unplugged (if you try to set up a new array through HexOS interface it will wipe the drives). If you know all your folders now you won't need to go into into truenas. I really think there needs to be a recovery feature in case of boot drive failure.
Here's a link to a thread with good instructions on how to recover, but it would work to "downgrade" as well, though kinda janky. I would assume cloning would work as well but haven't tried it.
https://hub.hexos.com/topic/3227-restoring-hexos-from-a-dead-boot-drive/