I'd suggest you prioritise defining a format earlier rather than later. The format / standard can evolve over time to accommodate the evolution of your product

I'd suggest you prioritise defining a format earlier rather than later. The format / standard can evolve over time to accommodate the evolution of your product (that backwards-compatible evolution can be defined as part of the first version) but to go from no standard to any standard is much more difficult once people are already using the platform: you'll either break established workflows, or burden yourself with a lifetime of supporting unbounded backwards compatibility (because every client will, by virtue of writing integration code, unintentionally define their own standard).