This is more than just data backup as we would need need to recover disk partitions/LVM metadata, boot records, etc. as well as all the data itself.
While other suggested, image-based solutions better fit your bit-for-bit requirement, you might also be interested in ReaR: https://relax-and-recover.org/
ReaR generates a bootable image which performs all that basic partitioning and is able to trigger an actual system/application data file restore using a variety of tools (including borg).
Slightly smarter approach is dd, then zero unused sectors and compress.
Both will produce an image which could me restored with DD (or mounted offline). Second will be smaller.
They should be run with unmounted partitions.
The most boring answer is "connect the disks to something else and use `dd` to copy the full blocks from start to finish into a file".
'dd' would have been my thought as well, I've heard of Clonezilla also but never used it and not sure it's really doing anything appreciably different.
I like the idea of 'dd' because I have a very clear mental picture of what it does. Just wasn't sure there was something else I might want to look at.