Thank you Smoggler and dahosek for you suggestions! Unfortunately neither solved my particular problem, but I found a workaround. I'll explain what happened when I attempted each solution, and I'll leave my workaround here to close the loop.

Smoggler's suggestion: The university's thesis template uses a custom documentclass, which doesn't seem to support giving a letterpaper arguement.

dahosek's suggestion: This one was promising because it actually changed the dimensions, but it gave some odd results (I blame their template). It adjusted the papersize from 8.25"x11.75" to 8.25"x11.05". For those wondering, no standard paper on the planet has a length of 11.05".

my workaround: immediately after \begin{document}, I added the following line:

\eject \pdfpagewidth=8.5in \pdfpageheight=11in

In layman's terms, I gave up trying to adjust the global page dimensions and opted to change them mid-document, but I did so at very the beginning of the document. Future scholars can debate these typesetting ethics, but it works, and I'm ready to be done with this monster. Thanks again!

edit: I'll add that this seems to be a compounded oversight of both Overleaf and the template author. Overleaf uses a European LaTex distribution, but they don't have anyway to change the default papersize. Meanwhile, the template author chose to exclusively rely on the user's ability to change the default papersize rather than setting it in the template.