By the way, I have two clients who have similar requests.
For them, we make completely individual client cabinets that are integrated with the OneBox API.
And, as Igor wrote, OneBox can be integrated with Bitrix24 as a transition process.
Here is one of the examples of our custom cabinets -
https://billing.hvosting.ua/True, it’s not a fact that this will work, because a custom client account starts at $10,000.