Answer
Start where business value and execution risk line up best. For some teams that is the interface because usability is blocking adoption. For others it is workflow automation, API enablement, or targeted code work because the real bottleneck sits behind the screen.
The strongest phase-one decision is the one that removes a visible pain point without forcing the organization to absorb unnecessary change all at once. Buyers should judge first-step options by outcome, dependency, and reversibility.