Like if I do the following

The dd command takes a while to complete, what if I decide that I don't want my machine to shut off after it's complete. Is there a way to somehow cancel that poweroff in there without prematurely terminating the dd?

You can make poweroff non-executable.

I tried ctrl-z with
sleep 22;echo x

It stopped the sleep without killing it, so I could fg it later, but the bad news is that it immediately executed the echo x (which would be the poweroff in your case).

I would check `which poweroff'. Then, ls -l it. In my case it's just a symlink to /bin/systemctl! So, chmod operates on systemctl! Which would work, but then you're messing with something *other than* what you thought you were (ls -lc)
So I sudo mv it to /sbin/powerof #one less f
ctrl-z gives the can't find poweroff error message. fg brings back your first command.
WHEN my long first command finished, then I mv'ed it back ok.

With newline instead of semicolon it seems to work:
The commands are queued up in memory.
Pause the current command with Ctrl-Z:
A "Stopped" message should occur.
Now continue the current process with the fg command:
The paused process continues.
And the following commands are gone.

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