When it comes to designing a multi-vendor (sometimes also referred to as best-of-breed) OSS architecture stack, there is never a truer saying than, “the devil is in the detail.”
Oftentimes, it’s just not feasible to design every interface / integration / data-flow traversing a theoretical OSS stack (eg pre-contract award, whilst building a business case, etc). That level of detail is developed during detailed design or perhaps tech-spikes in the Agile world.
In this interim state, I often leverage use-case design and touch-point mapping through the stack to ensure that all of the use-cases can be turned into user-journeys, process journeys and data journeys. This process can pick up the high-level flows, but more importantly, the high-level gaps in your theoretical stack.