@florjanssens so this thread doesn’t get lost, as I am also interested in other opinions on this.
On the compression side, as usual it will depend on what you are trying to compress. Assuming, most of your images are the RAW format, you can achieve quite a bit of compression levels, whereas jpg would not give you all that much.
While ZSTD is now available with btrfs, I don’t think we offer it in the current stable version of Rockstor’s WebUI. But that’s coming. @phillxnet care to comment on this area?
Same goes for the LUKS piece, here’s a thread you might have already seen, albeit it’s a bit older.
For the migration (and then subsequently using the old machine as backup) you could resort to using ‘rsync’. Any solution will take some time, since 30 TB are not a small amount of data.