There's no way to delete all documents from an engine in one API request. You'd need to work with the Documents API: Documents API | Elastic App Search Documentation [7.15] | Elastic.
Just a thought, if you need to regularly delete all documents and recreate them... you might be able to use Meta Engines.
You could create a Meta Engine that is backed by a Source Engine. Load all of your documents into the Source Engine, and whenever you need to delete all documents, delete the Source Engine, recreate it as a new engine, and load all documents.
You'd keep all of your relevance settings in the Meta Engine, etc. so that it is still retained when you drop the underlying Source Engine and recreate it.
Just a thought.