Thanks @DavidBoike.
I think using a Transactional Session here still misses a very potent benefit. Simply doing the “regular” database interaction manually would put the responsibility for concurrency control back into the hands of the developer, whereas sagas take care of that for us. Sagas are far less error-prone, and easier to grasp, when it comes to concurrency control. That is why I would like to use a saga, not just further down the chain, but also at this point.
In essence, triggering the saga could be as simple as sending a message. However, that would introduce the overhead I described. I also just realized that it would introduce the risk that a queue full of unrelated work might cause the HTTP request to not get handled in time. For these reasons, I would want to handle the event instantaneously (and locally).
Is there an alternative way I could trigger the saga and benefit from its features?