Hi,

You can start with WordPress or another CMS if you want to build the 3rd party payment integration in PHP. You can use a WaaS to create the white label website builder and add your own PG via WooCommerce.

https://www.ecwid.com/partners/white-label has a white label service and has a lot of integrations, or perhaps this: https://simvoly.com/whitelabel-website-builder

Though to be fair the first option has hundreds of payment plugins though Woo, so unless it's your own crypto, you might find there is one already available for your payment gateway.
https://woocommerce.com/product-category/woocommerce-extensions/payment-gateways/

If you want any further information or help on your project, please get in contact and I'd be happy to try and find a solution with you.