On top of that, I'd consider adding a bit of idempotency to the form submission -- to make sure that if the request did in fact succeed even though you did not

On top of that, I'd consider adding a bit of idempotency to the form submission -- to make sure that if the request did in fact succeed even though you did not get the confirmation of a success, subsequent retry/resubmit attempts won't result in duplicate entries. A rough way to do this would be to add a hidden UUID value in the form data that is generated once up front and used by both the client and the server to ensure that the result of the first successful request can be cached and re-returned.