If you inspect with $variables['items'][0]['content']
with Devel kint
kint($variables['items'][0]['content']);
But it doesn't work and I think it's because by the time I get to that point, Drupal has already used the uri for the #title.
Correct, so all you gotta do is check if the title is equal to the url.
if ($variables['field_name'] == 'field_event_location_link') {
$link_url = $variables['items'][0]['content']['#url']->toString();
if ($variables['items'][0]['content']['#title'] == $link_url) {
$variables['items'][0]['content']['#title'] = 'Location';
}
}