I am currently rebuilding from scratch a product catalogue (to feed a shopping website).
The existing legacy system is heterogenous, Long story short : The some products are stored in a FileMaker database (with a big spaghetti data model) and other products are edited as YAML files ...

So my question is : what are the strategies for data modeling if I want to go step by step migrating to the new system, that is to say, do I have to make an exhaustive data model as soon as possible (different kinds of products, handle taxonomy, products variants, stock management, ....) or can I begin small, only modeling a particular kind of product and growing my data model as I'm adding more products types ?

The advantage of begin small is that I don't have to take account of all details at the beginning (do something that work - minimum viable product) but in the other hand it can mean that the choices made earlier can stuck me in the long run.

What would be the most effective approach?