/mnt/Storinator/NASty/Archive

Can I compress my media to back it up to a separate drive?

Yes and no.

Yes : You can compress it but the quality will get worse

No : You can compress it without quality loss but you’re going to save less than 3%

I’m aware of 3-2-1 only by name but not sure what is the best way to proceed in my current circumstances.

If you want 321 then it might be easier setting up another RPI at the friends house with the backup disk since the majority of data is already on there. Then you can upload to backblaze at your leisure