Most companies build their own billing solution or use off the shelf solutions like Chargify, Chargebee, Recurly etc... but I am missing multiple payment methods like WebMoney, wire transfer (manual), cryptos (bitcoin, ethereum) in them. These service providers end up having your data.

Why must every entrepreneur go through all this pain?

There is Killbill.io but it's in Java and quite deep. It does not have many payment plugins. I am finding it difficult to hack.

Currently, we use WHMCS but it's difficult to customize its look and feel to match our SaaS app. It solves every billing issue but it's unable to offer seamless integration with SaaS+SPA app.

It has many payment plugins https://docs.whmcs.com/Payment_Gateways and it's quite easy to develop your own, see: https://developers.whmcs.com/payment-gateways/

So in terms of hackability, WHMCS scores above Killbill.

Most product and services are moving to subscription-based billing and this space does have some friction as evident from the fact that I am forced to use WHMCS which is built for hosting industry. And it's the only product which solves all my problems except one. (seamless integration)

I want to build a self-hosted app based on GitLab's business model and I am interested in sponsoring work in this direction if anyone is interested, please drop me an email at istartup@protonmail.com

Please let me know your thoughts on this.