At our startup, we chose to start with a monorepo. Our team is small, but one of the big advantages we’ve had so far is avoiding the n*m (for n services and m tools) problem with dev tooling - which leads to a very smooth developer experience.
For example, to run one or more services locally, we use a single script that sits at the repo base - ‘dev.sh service1,service2,...’. This avoids a lot of headaches for our developers, as we enforce compliance when adding a project to the repo. Lint config? One to rule them all. Test coverage thresholds? Single one. This consistency is the biggest win in my opinion.
Similarly, our integration tests are very easy to write without commit skew.
Finally, sharing libraries has been painless - since we have common/ and common/third_party/ directories at the monorepo root.