A company I worked with got most of its cash from one customer. So most of the engineering went to that customer too. Custom workflows, custom rules, a machine set up by hand just for them. Every other customer got what was left.
Then the whale fell months behind on paying. The platform had been bent around them, the team was exhausted keeping their work running, and the money that was supposed to fund the fix was sitting in their accounts payable.
A whale client feels like safety. It is the opposite. The more you custom build for them, the more your product becomes theirs and the less it is yours to sell to anyone else.
Do the work. Charge for it. But build the general thing and let them be the first customer of it. And when you feel desperate for their next check, that is the signal to go find the second customer, not to say yes faster.
-Christopher