Looking for a CRM that stores leads by client or event – and that the client can access but only see his leads, and not another client’s

I need a CRM to manage client leads, so I don't want one massive list of leads, I want to able to store them by client. I also want that client to be able to access his leads, but not see the leads of another client.

We run events for clients so basically need to manage the customers who will be attending that client's events.

Would also be nice to be able to create campaigns and store leads that way. So basically if I could have a folder for each client, then a subfolder for that client's event, and then the leads/customers are stored in there…. That would be great.

Monday.com seems to fit this, but we're hoping for something more customizable to our company's branding.