I get “Class … does not exist” after setup:di:compile, what now?

That’s a dependency-injection / generated-code problem. The usual causes are a stale generated/ directory, a bad preference or type in a di.xml, a proxy or interceptor that never compiled, or autoloader cache pointing at a deleted class. We do a clean rebuild, remove generated/, recompile, redeploy, identify the module that introduced the bad wiring, and fix the config rather than just clearing caches until it works by luck.