I think in this case a well defined modular monolith would be a excellent starting point, especially if you need to (1) build from zero and (2) hit the ground running with a small team (without all the extra things you need to pay attention to and create solutions for when going the microservices route).

The great thing about the modular monolith is that it gives you a fantastic foundation to grow: You can still invest in multiple teams each handling a certain part of your problem domain (due to the modularity); and because it is modular you could easily break it apart, and evolve it, into a microservices architecture if and when there comes a time you need to do so (due to extensive problems getting code shipped, or difficulties concerning NFRs defined for the architecture, for example scaling issues in production).