Hi
I have just used 2 weeks to fine out that it is really difficult to fine a Onepage Checkout plugin that works with our payment gateway AND this theme!
The best plugin is: https://wordimpress.com/plugins/woocommerce-quick-checkout/
Unfortunately, does this plugin not work with your design theme, the built-in Cart in the top menu can not be updated by the plugin.
I hope that you could make the theme compatible with this plugin: https://wordimpress.com/plugins/woocommerce-quick-checkout/
Best regards
Tonny