I built a Proof of Concept (PoC) to explore the idea of manage and scale dynamic data modelling in multi tenant saas. Catch it here:

a-smarter-way-to-handle-dynamic-fields-in-multi-tenant-saas

I also think GraphQL would be a better choice here. But at the end of the day they need to gel with OutSystems and not negate the rapid development benefits it brings.