Sigh, it was a f-ing PSO cache issue. Have never had this happen before, somehow it was attempting to retrieve an invalid cached PSO. Deleting the cache didn't seem to help though, had to do a GPU driver update and a full project reimport. I love not having helpful error messages
One of the worst things about Unity people don't often mention is how bad its error and crash reporting can be and how utterly impossible it can be to diagnose an error without source code or even a coherent error message. (Or in my current case literally no error message at all)