As compared to SR as Einstein originally developed it, yes. But note that modern usage of SR often makes use of the same differential geometry tools that were developed for GR--because those tools turn out to be useful even in flat spacetime.

> But once SR was clear it was "just" an extension to the same ideas.

Again, if that were the case one would not have expected it to take ten years to get to GR. Yes, Einstein had to learn about Riemannian geometry from Grossmann, but that didn't take him anywhere close to the ten years to do. He spent most of that ten years struggling with the physical concepts involved, not learning the math.

> The leap here was the original idea that gave rise to SR. The rest could have been worked out by most physicist of the time.

This is strongly contradicted by the fact that there were many other physicists working on a relativistic theory of gravity at the time--and they all got it wrong. Only Einstein, and Hilbert (who was a mathematician, not a physicist) after getting key input from Einstein, got it right.

The necessary transformations for special relativity (Lorentz transformations) were already developed by Hendrik Lorentz and Henri Poincaré. Einstein's central contribution for special relativity was to reinterpret these equations in a novel way.

But he did that only after having Einstein visit him for several weeks in the summer of 1915 and having long discussions with Einstein about the latter's ongoing efforts to discover a general theory of relativity. AFAIK Hilbert himself acknowledged that his final effort was only possible because of all the work that Einstein had already done.