If you're looking for something to maintain the contacts and the workflows for each contact, I'd try something like Tonkean.
Tonkean will work will you to define you workflow for each customer.
As an example, we use the free version of Hubspot to host our contact information for each of our customers. We then have workflows set up for them in Tonkean.
For example, when a new contact requests more information on our site, Tonkean forwards this information to Hubspot and a new contact is created. Then, we have Tonkean set someone on the Sales team as the owner, you can set parameters so it knows who should be assigned to which type of customers. Then a workflow beings, the Tonkean Bot will reach out the the relevant people when action needs to be taken or to the relevant business tools and application if there's no people involved. For example, Tonkean can tell Mailchimp to send a welcome email and tell Salesperson A to set up an initial call.
Tonkean is basically a workflow management tool, but it works great for freelancers, support teams, agencies - basically anyone with a hefty to-do list. All the time spent on managing what you're working on, not actual work, can be left to Tonkean. That way you can focus on the things you enjoy doing as a freelancer, like building client relationships or working on your brand expansion, etc.
I definitely recommend you check it out. https://tonkean.com/rpa/