For this specific need, just once. Disk is 1TB but only about 350GB used.

'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.

If you need multiple version of such a disk image, a tool like restic (or I guess borg too, not sure?) can also compress what's provided to it via stdin. So you'd dd directly into restic and it will delta compress to earlier backups.

Yea; this was the heart of my 1/10/1000 question. Once? I'd probably just use dd and call it a day. 10 times? Probably download clonezilla. 1000 times? Probably automate something w/ restic and some kind of object storage layer so I don't just have a directory full of giant images/deltas somewhere.