Posting for posterity: I have solved this issue using the following IIS configuration -
IIS → Click the website in question → click Request Filtering → click the Headers tab → click Add Header → Configure this screen using the instructions below:
In the “Maximum allowed content length (Bytes)” field, enter this number, or something much larger than what is already in there:
2147483647
That fixed the upload size issue for me.
Some back story: When I originally posted this, I had ruled out IIS being the problem, stating that I had isolated the problem to Caddy - in fact, the large upload worked fine while no SSL was engaged. Once the SSL engaged, IIS began presenting this problem, and since I get my SSLs from Caddy, I wrongly assumed the problem was with Caddy.