nfs over the internet is usually bad idea. Network delays is killer for NFS. You can use ssh instead. And add some mbuff for better results.

I've done similar things with ssh pipe tunneling and dd ala:

,,, dd if=/dev/sda | ssh user@host "dd of=/some/other/file/or/disk" ,,,

Should really do it while the root is ro, but in a pinch this will mostly be ok on a quiet machine.

You might be able to run rsync afterwards to spot check if you are paranoid.

If you didn't need block level I would have suggested just using rsync or rclone.

I only use this really to get a free public ipv4. (Since not all clients support ipv6 only) Is there any other free/better way to get an ipv4?

I've never had an Oracle Cloud server. However, I do image backups of the server once a month, then test them in a VM, After hearing so much about Borg, I finally pulled the trigger and ran it from the cli for a while. Then I deployed Borg UI, and that's pretty amazing. I used to use Duplicati, but while Duplicati seems great at making backups, it's not so keen about doing restores. At least, that's my experience.

What changes are they making to always free?

Yeah that's a reasonable way to take a backup. I mean I would have recommended an actual backup tool, but dd is perfectly good. Especially since you zeroed as much as you can, since you didn't have a tool skipping unused blocks.

But the Wi-Fi bit was pretty gross, yeah. Fine to test but never ever for something I actually wanted to use.