I'm pretty sure the answer to my question is that tRPC is not suitable for this project, but anyway I would like a second though and perhaps also some inspiration to what kind of projects it is suitable for.
I'm soon bootstrapping a project where I'll have multiple webapps (NextJS) and possibly iOS/Android (React Native) which consumes API(s) (Express/Fastify?) that talks to one or more databases (Prisma) and third party APIs. The API/functionality is fairly large, so it would make sense to not make a huge monolith but aim for something between microservice/macroservice architecture. The API also needs to be "public", so I've had a look at trpc-openapi.
Given this, I've thought about solving it like so:
"Consumer facing apps" -> BFF -> APIs -> Databases
Where would tRPC fit in this solution?