Of all the commonly used languages, C++ is the most complex, baroque, and filled with subtle and not-so-subtle footguns. Of course with all that downside comes the upside that C and C++ are the lingua franca for low level programming where performance matters, not only HPC (yes, I hope Rust will be there one day, but I digress..). Fortran is pretty much limited to numerical computing, you're not going to find much Fortran libraries outside that domain.

So if you envision doing programming like that outside HPC after your PhD, C++ might be worth the investment.

But of course a lot comes down to what language other people in your group and your particular subfield, and what your supervisors are using. Doing a PhD is hard enough without being the odd guy out who's using a language nobody else in your community is using.