as a mechanical engineer i wrote lots of FEM codes in FORTRAN, i still do and honestly enjoy programming in it. the lack of graduates who know Fortran is a correct estimate. As far as I know, it is usually replaced by MATLAB and Python in our departments. Unfortunately, C++ is taught less.
i don't know much about future CPU technology or why it is likely that Fortran codes don't perform properly on GPU. but i know that NVIDIA takes too much time to provide CUDA-FORTRAN for windows. nothing happened after 4 years.
But it seems to me that the isolated Fortran programmers have recently become well connected. Look at the website [1], Package Manager or LFortran [2].
[1] https://fortran-lang.org/en/
[2] https://lfortran.org/
that's great even for not-so-complicated scientific projects. keep in mind that lots of commercial software in computational engineering are rely on FORTRAN for user defined sections and tweaking. for example ABAQUS in FE analysis.
I don't have an answer, but I feel the same when I try to extend my knowledge in interdisciplinary areas. Have you ever heard people say they love the beginning phase of relationships? I am like them with learning. I enjoy the introductory parts and primary levels, but I always struggle through advanced sections, so I can't apply anything I learned from those references in my studies.
I'm tired of these kinds of titles. They are like news headlines and nothing but a way that maybe it worked for someone. as the adage states: "there is no royal way for learning."
In my experience, writing down doesn't help me develop new idea, but exploring new ways to support it or explain process as straightforward as it should be, which I can not do in my head or real-time conversation. As the author says, it takes lots of time and effort to put yourself in a stranger's mind.
No one can have an enjoyable and helpful reading experience by picking a textbook or applying for a random course. Instead, reading the history of mathematics to learn about topics and their background would be a great way to capture the idea and a good starting point to find your favorite area. I recommend searching for Paul Lockhart's books, Mathematics in Western Culture by Morris Kline, and Famous problems of geometry and how to solve them by Benjamin Bold.