Thanks for the response! I guess I didn’t want to go through the process of disabling 30 plugins and retrying them one at a time.
Sometimes I forget that this stuff isn’t magic.
Anyway, I did exactly that and found the culprit plugin to be Cornerstone. A thread on the X Theme support forum made a suggestion to bump the memory limit up to 256MB. Once I did that Cornerstone stopped causing the site to 500 error (So far). Kind of insane to me that a WordPress plugin could need a quarter of a gigabyte of RAM but whatever, it’s working now. Thanks!