Error correction and compression are usually at odds. Error correction usually relies on redudant data to identify what was corrupted it also helps if the process for error correction is ran more frequent. So storing it away offline is counter to the correction and the added redundancy will reduce the space gains. You can check different error correction software or technique. Ex RAID. I recommend following the 3-2-1 data backup rule. Also even if you can’t do all the steps doing the ones you can, helps.

Sidenote optionally investigate which storage brand/medium/grade you want. Some are more resistant than other for long term vs short term. Also even unused storage will degrade over time whether the physical components, the magnetic charge weakening or electric charge representing your data. So again offline all the time isn’t the best; run it a couple times a year if not more to ensure errors don’t accumulate.

Sadly I won’t give specifics because I haven’t tried your use case and I am not familiar, but hopefully the keywords help.