Pedro - I’ve found the “classic” approach of working from the use cases to the functional requirements to work well for me. Use cases capture the different ways in which a system is used. Each use case implies a number different functional and eventually technical requirements.
So - starting from the top: capture and describe the use cases, derive the functional requirements from there (supplementing it with non-functional requirement such as scalability etc).
Use these functional requirements to map to technical requirements and a task breakdown in your task management or project management system (ie Jira).
If often find it useful to document the flows through the system - both as state machines and activity diagrams - as supplemental sense checking artefacts.
The architectural views - ie the deployment, logical, process and implementation views are useful lenses with which to examine the architecture. Also consider the static and dynamic states of the architecture.
Once you approach the implementation phase I strongly recommend looking at this: https://12factor.net/
It provides a very useful methodology to apply to creating the application.
Merci Jacques! I coincide that this traditional modelling can fit well in this process. And I will have now a look at 12factor and see if valuable at this stage. Thanks again! Pedro