Why openers matter separately from length
Sentence-length variation is the better-known signal, and it is the one detectors respond to most directly — it has a name, burstiness, and a tool of its own. But length is not the only way prose becomes monotonous.
A paragraph can run 8, 24, 11, 19 words — genuinely varied — and still open every sentence with a determiner and a noun. The reader feels the sameness without being able to point at it, because the thing repeating is position rather than rhythm.
What to do with the result
The usual fix is not to hunt for synonyms. Start one sentence with the evidence instead of the claim, or with a subordinate clause, or with the object. Changing what the sentence leads with changes the opener as a by-product, and it improves the argument at the same time.
The same principle underlies the worked example on humanizing an AI-written essay: variation comes out of the structure of what you are saying, not from applying it to the surface afterwards.
Writing in your own voice is the longer argument behind all of this: variation that comes out of what you are saying rather than out of a thesaurus.
One thing this tool cannot see is worth checking by eye at the same time. Repeated openers make a document monotonous; inconsistent register makes it read as assembled from more than one source, and that is the shift an experienced marker notices before any software is involved. A formal sentence, a conversational one, then a formal one again is a stronger signal to a human reader than any opener count.