> we are considering an in-memory emulator to open source ourselves
I'd suggest a persistent emulator, using something like SQLite (one row per record). Even for local development, many applications need persistence. And it'd be even enough to run a single node low throughput production server which doesn't need robust durability and availability. But it still has enough overhead and limitations not to compete with your cloud offering.
What's however important is being as close as possible to your production system, behavior wise. So I'd try so share as much of the frontend code (e.g. the GRPC and REST handlers) as possible between these.
I'd suggest a persistent emulator, using something like SQLite (one row per record). Even for local development, many applications need persistence. And it'd be even enough to run a single node low throughput production server which doesn't need robust durability and availability. But it still has enough overhead and limitations not to compete with your cloud offering.
What's however important is being as close as possible to your production system, behavior wise. So I'd try so share as much of the frontend code (e.g. the GRPC and REST handlers) as possible between these.