First thing it did was overwrite the partition table and everything else with that, to make its own, since it could disregard all the existing data.

I agree with the other commenter, commercial recovery, if the data was that crucial.

I haven’t used raidz but a quick search tells me it supports single-drive expansion.

Maybe reconfigure your raidz as a 2-drive system, then copy over all your data into it, and expand it back into a 3-drive system after.