How do I know when I've outgrown the manual stack?

Three signals. The commission spreadsheet has more than three tabs that all need to stay in sync. Manual reconciliation takes more than two hours per week (which means roughly an entire founder afternoon every week). You've made a commission error that affected distributor trust, even a small one. Any of those three is enough; combination of two or more is a clear signal to evaluate SaaS in the next month rather than letting the manual stack run another quarter.