A third-party extension is throwing a fatal, can you fix that?

Yes. A vendor extension throwing Class … does not exist, a fatal in a plugin/observer, or a bad di.xml after install or upgrade is one of the most common 500 sources. We isolate it with bin/magento module:disable bisection to confirm exactly which module is at fault, then either patch the conflict, pin a working version, or quarantine the module and report it to the vendor, whichever keeps your store up safely.