I get a 500 Enternal server error when I try to import the demo content. My php is configured as suggested in the documentation
Are you referring to this documentation - https://themeisland.ticksy.com/article/3794/
There are several configurations that could be at play, including php version, php limits, and host limits - in addition to any third party plugins you might have running.
I recommend first working with your host to ensure you have increased your php limits according to the article posted. From there, check your php version, and finally, your plugins.
Importing the content is not a theme specific function, rather a native WordPress import process, so there is not much I can offer beyond that. A 500 error indicates your host as the bottleneck. If you are having issues with your host, I trying a WP specific host like WP Engine, since host configurations do not always support various WP functions.