Random reviews are great in theory, but since PHP is a server-side language (not client-side like javascript) any caching you do on your website to speed it up will make the randomness no longer work.
This is why Site Reviews does not provide a “random” option.
If you want to do this anyway, here is a custom shortcode snippet that may help you get started: https://pastebin.com/naUae8zB
A more advanced solution would be to write some javascript which uses AJAX to get the reviews on page load, using the glsr_get_reviews() function in the PHP callback function to order reviews randomly (using the orderby option in the function).
