Hi Lazy Bee,
500 internal error analysis can be frustrating, but you can try this quick checks;
- Insite one server with error, and inside of IIS Manager browse the SharePoint web application where you encounter the error, get a detailed error message to identify the specific issue in the SharePoint Web Config file.
- Check if Web App Pool is stopped, if yes, start it.
- You can also reset IIS.
For forensics analysis, you need check all event logs after your update. can be hard, but can be the way.