This Magento module will send users with a valid cart to the Rapidez checkout instead of the Magento checkout so Rapidez can be used as a standalone checkout.
composer install rapidez/magento2-standalone-checkout
bin/magento module:enable Rapidez_StandaloneCheckout
All you need to do is set the Rapidez url in the standalone checkout config.
You can do so under Stores > Configuration > Rapidez > Standalone Checkout > Rapidez Url
Rapidez' Registration feature will not automatically log you in to Magento
By default Rapidez shows a full header and menu on the success page, you might want to consider showing the limited header instead.