UPDATE:
I used sudo apt-get install gromacs on Ubuntu 20.04.1 to get the precompiled binaries of gromacs 2020.1 (different from version 2020.4 as I was using above). The gmx binary also has an identical hanging issue between these two different versions and build methods. I also did a fresh re-installation of the OS, with no improvement.
I also tried rebuilding version 2020.4 again with gcc-8.4.0 and CUDA-11.1, as well as just gcc-8.4.0 with graphics card support entirely turned off, so I am fairly confident it is nothing to do with the graphics libraries/card