Ah, I think the issue is that my personal site doesn't have a pre-existing media bundle of type image. I only recently enabled Media (and that was only to make a Remote Video bundle).
So I guess installation of this module needs to create that bundle if it doesn't exist?
I did not get this error when installing on top of Umami.
I'm assuming the issue here is some collision between the fields that this module is creating and my pre-existing fields.
I'm also getting WSOD errors as I navigate around the site after the attempted installation.
Symfony\Component\Routing\Exception\RouteNotFoundException: Route "imagecache_external.admin_settings" does not exist. in Drupal\Core\Routing\RouteProvider->getRouteByName() (line 211 of /code/web/core/lib/Drupal/Core/Routing/RouteProvider.php).
stevector → created an issue.