Is there some way in Stash to say that I've completed reviewing the code?
There is the approve button, but that is only useful if you're happy with the code.
However, if you want to let the developer know that you've finished reviewing and that they can start implementing your changes, there is no way to do this, other than leaving a comment saying "Finished reviewing".
Otherwise, the developer will end up pushing up lots of tiny commits for each comment, when it's usually nicer to bundle the changes once the reviewer has finished.