Hello,
1/ Our theme is compatible with PHP 8.0, maybe one of your plugins is the reason for the problem.
Disable 3rd party plugins one by one and check if there is a 503 error then.
Also, contact your hosting provider support to check if there is any issue on the server.
2/ Have you read this article – https://wpmudev.com/blog/w3-total-cache-settings/ ?
At first sight, we do not see any styling problem in the mobile menu on your site, right now.
Could you provide a screenhot of the problem?
Try to disable/enable different settings of the cache plugin to find our which one causes the problem for you.
Regards