Ok. that is not simple to do at all.
I would rethink the whole menu. Do you really need so many items? Could be overwhelming for users. Maybe instead link to a category or tag that contains group of posts?
But to offer a solution: you could create your own dropdown to put on a simple HTML sidebar widget using html:
<form name="mymenu" action="" method="post">
<select onchange="window.location.href=this.value">
<option value="" disabled selected>Menu</option>
<option value="https://isitshakespeare.com/">Home</option>
<option value="https://isitshakespeare.com/the-definite-2/">The Definite 2</option>
<option value="https://isitshakespeare.com/the-usual-suspects/">The Usual Suspects</option>
<option value="https://isitshakespeare.com/the-chandos/">The Chandos</option>
<option value="https://isitshakespeare.com/the-minor-suspects/">The Minor Suspects</option>
<option value="https://isitshakespeare.com/the-weird-suspects/">The Weird Suspects</option>
<option value="https://isitshakespeare.com/the-missing-suspects/">The Missing Suspects</option>
<option value="https://isitshakespeare.com/the-unknown-suspects/">The Unknown Suspects</option>
<option value="https://isitshakespeare.com/the-fakes-and-forgeries/">The Fakes and Forgeries</option>
<option value="https://isitshakespeare.com/the-friends/">The Friends</option>
<option value="https://isitshakespeare.com/the-critics/">The Experts & Critics</option>
<option value="https://isitshakespeare.com/the-institutes-owners/">The Institutes (owners)</option>
<option value="https://isitshakespeare.com/the-science-of-proof/">The Science of Proof</option>
<option value="https://isitshakespeare.com/the-possessions/">The Possessions</option>
<option value="https://isitshakespeare.com/the-missing-limning/">The Missing Limning</option>
<option value="https://isitshakespeare.com/the-timeline/">The Timeline of the Face</option>
<option value="https://isitshakespeare.com/the-search/">The Search</option>
<option value="https://isitshakespeare.com/the-reason/">The Reason</option>
<option value="https://isitshakespeare.com/the-private-collection/">The Private Collection</option>
<option value="https://isitshakespeare.com/the-words-of-will/">The Words of Will</option>
<option value="https://isitshakespeare.com/the-links/">The Links</option>
<option value="https://isitshakespeare.com/the-funding/">The Funding</option>
<option value="https://isitshakespeare.com/contact-me-with-something/">Contact me with something good !</option>
</select>
</form>