How can I track the progress of dd after I've already started it?
Yes, from a separate terminal you can issue this command:

kill -INFO 1234


where you need to replace 1234 with the pid of your dd process. You can look it up with the ps command.

Another simplification at a second terminal:

sudo kill -s siginfo $(pgrep ^dd)   # get dd info


Another simpler way is to request the progress information from the same terminal as dd by pressing Ctrl-T.