I don't know about your specific example, but doing an operation where you rewrite most of a multi terabyte database online is almost certainly not best accomplished with SELECT/UPDATE. Even if you need multiple passes, that's N terabytes times M passes times two. That's... not a lot of reads and writes. Dumping the database to files on blob storage, rewriting them, then reading them into your destination is almost certainly the fastest and cheapest way to go about that.

And that's not an iops avoidance thing, that's a "this isn't what your database is built to do with the configuration your running it in" sort of thing.