After a donor shard transitions to kPreparingToMirror, it should write to the oplog that it is now converting all future writes to distributed transactions. This is so that all recipient shards can read this message. These recipient shards will know that they are done applying by reading this oplog message.

The oplog message should have an o2 field matching reshardFinalOp (bonus points if you pull out that magic string into a global variable), and should look something like this.