From above description it seems like you want to manage your customer tenant separately without impacting services and billing of your tenant.
Please do correct me if this is not the ask by responding in the comments section:
You must consider using Microsoft 365 Lighthouse. Microsoft 365 Lighthouse helps Managed Service Providers (MSPs) grow their business and deliver services to customers at scale from a single portal. With Lighthouse, you can standardize configurations, manage risk, identify artificial intelligence (AI)-driven sales opportunities, and engage with customers in an unprecedented manner, allowing you to anticipate customer needs and maximize their investment in Microsoft 365.
- You could manage multiple customer tenants from Tenants page in Microsoft 365 Lighthouse.
- Similarly you could manage users from Users page in Microsoft 365 Lighthouse.
Please "Accept the answer (Yes)" and "share your feedback ". This will help us and others in the community as well.
Thanks,
Akshay Kaushik