I use a in-memory database per unit test with both rocksdb and sqlite, it is a game-changer to get better quality tests.
Overall premise is wrong though. Moving the database out of process will change performance characteristics and data architecture too much and will cause massive headaches at exactly the time when you are trying to scale with success. You should have out of process performance tests early to catch these issues, even if you do deploy a single node.
If success can be satisfied with a single node and you are satisfied with availability and recovery that gives you then great, but it isn't all I need.