CRM with a customer “Tab”

Looking for CRM that has a specific feature of allowing to accumulate multiple billable requests per client+project and then invoice the client on monthly basis for services rendered. Basically, rather than doing fixed invoicing for X ammount of services manually, have an ongoing rolling tab for each client.

Our business involves small pool of clients with which we have multiple month long project-based partnerships and large proportion of services is rendered ad-hoc on request. The client basically gets an estimate at the beginning of a project, but final services and costs may differ significantly. Im struggling to find best CRM solution to facilitate this as most are focused on mass spamming and selling individual products rather than managing ongoing partnerships. Could use some advise from those with similar use case?