With a high fraction of renewables, the reverse is probably better in the long run. The larger geographic area you connect, the less you're affected by weather systems, and the wider area you can draw dependable dispatchable power such as hydro from. But that depends on having enough grid capacity to move enough power around, which is currently a problem.

But I wonder from a reliability (or lack of cascading failures) point of view whether synchronous islands interconnected with DC interconnects is more robust than a large synchronous network?

It's hard to build big generators, so as we already struggle with infrastructure I don't think that's feasible, but it would be great.