Starting with monorepo is a wise thing to do — after all, that’s the simplest model to work with and it has potential to greatly reduce amount of duplicated code the teams may produce.
From my experience, most successful DBT deployments has to evolve to multi-repository model at some point.
One of the drawbacks of splitting repositories into multiple ones is the lack of global lineage graph and common documentation. This is a huge selling point of DBT and it would be a shame if we could do nothing about it.