Hire the engineer. The internal-tools platform is a six-month detour at best; an additional senior compresses the roadmap by approximately seven weeks within their first quartera, and removes the on-call failure mode that has cost two engineering weekends in the past sixty daysb.
The platform investment becomes correct only after a third hire. The diminishing-returns curve on raw engineering capacity does not bend until coordination overhead exceeds throughput, and that point is not yet here. Until it is, every additional engineer is worth more than every additional toolc.
Move on the hire this quarter. Reserve the platform for the quarter after the third engineer signs.