How do you design software for reuse across products?
Answer
By building a platform of blocks rather than a series of projects. Over the years I developed the Lemmid platform exactly this way, designing for reusability, flexibility and extensibility so that a new product can be assembled quickly from parts that already work. The related question of monolith versus microservices has no universal answer: the right architecture depends on the size and shape of the thing you are actually building.