Depends on what you are envisioning in terms of an LLM engineer.
- Making advancements in algos & systems
- Building AI systems that people will use
Certain things will be helpful to both
- Math, in particular prob/stats & lin alg
- How to do correct & reproducible experiments
This is one of the resources I used when I taught a flipped ML course when deep vision models were just coming out for the first time. It might seem old or irrelevant, but this is certainly the baseline theory that still underpins today's and tomorrow's techniques.
https://work.caltech.edu/telecourse