This looks like a nasty bug when device removal (resulting in an indirect vdev) is used in the presence of active block-cloning.
A patch was made into OpenZFS, but it only prevents future crashes. If you already tripped the bug, then the patch might not be enough to get your pool back online.
Here is another thread for reference.
EDIT: If you’re able to import it into an Ubuntu live session, you can share additional information about the pool.
zpool get all Server
zpool history Server | grep "remove\|detach"
It’s possible that this is a different bug and might only relate to a corrupt or failing boot device that TrueNAS resides on.