Hello,
We limited the depth of the mobile menu because in amp version it is not yet possible to make good open/close actions for menu items that have submenus and all of them are shown at once.
We can propose you provide us admin/ftp details and we will add filter (it will also be included in plugin update) and few lines of custom css
The result will be next -> https://prnt.sc/37Zd7NrLUI9m
Waiting for your reply.
Create an account via Dashboard (Users > Add new) with administrator role http://prntscr.com/s3rc9m > provide us with username and password via the Private Content area http://prntscr.com/s5mao7 You can use any email for it.
If you do not know how to create FTP contact with your hosting provider, they will help you.
Regards