Ask HN: Best CRM to use for a B2C startup with 1M users

I am currently working on a project where the client has used WHMCS as their main CRM - it handles all their invoicing, subscriptions, add-ons, etc. The system was developed approx 7 years ago and while it was OK back then, it is very painful to work with now with almost a million users. Even basic features like account upgrade/downgrade, addon addition/removal, etc. are very hard to manage.

Any suggestions for a scalable system that handles all this and can integrate nicely with other services (e.g. Mixpanel, custom BI tools, etc.)