Interesting journey. Here in Australia one particular software package (pracsoft) had ~80% of the market (a decade ago when I last programmed against it) for general practice doctors, backend is MSSQL, and it's easy enough to tap into the calendar table for a service like this.

Also agree with your conclusion that it's dangerous to build a SaaS that relies on platforms that are not open by default and which you have no control over (and especially when the platform provider or solution provider is potentially a direct competitor).

Good lessons learned and interesting read.