How do you prevent early high-paying clients from pulling your SaaS into custom agency-style work?

When a few large accounts signed up, we were thrilled. But soon, their custom requests began dominating our development sprints.

We caught ourselves spending 80% of our engineering time building hyper-specific workflows for 20% of our revenue. It kept them subscribed, but it completely stalled our roadmap for the broader market.

For founders building self serve SaaS products: how do you draw the line between accommodating large customers and keeping your product scalable for everyone else?