I would use the PHP Snippet provided here:
https://docs.generatepress.com/article/generate_post_date_output/#only-show-updated-date
This code will only output the most recent date.
There are two lines you can edit to add your ‘prefixes’:
The first line is for the Published date:
$time_string = '<time class="entry-date published" datetime="%1$s" itemprop="datePublished">%2$s</time>';
You could add ‘Published on’ for example like so:
$time_string = '<time class="entry-date published" datetime="%1$s" itemprop="datePublished">Published on %2$s</time>';
Now the next line is this:
$time_string = '<time class="entry-date updated-date" datetime="%3$s" itemprop="dateModified">%4$s</time>';
And to add Last updated
you would change it to this:
$time_string = '<time class="entry-date updated-date" datetime="%3$s" itemprop="dateModified">Last updated %4$s</time>';