When you only have a handful of clients, even the shittiest one matters. Setting standards for what you will or won’t put up with this early may not be optimal. Getting the framework in place internally for your team to identify the problem clients, keeping the toxicity transparent and distributed through proper heatsinks will lead to the more organic evolution of what you will or will not put up with from a client when money is involved.
So part of the framework could actually mean de-prioritizing the client’s work internally, while prioritizing communication and expectations for some time. You could also boost morale of your team by requiring sign off from leadership on every little deliverable to show that you acknowledge the problem and those managing it internally.