The real trick is not over complicating things. The goal is to have high fidelity rendering where the eye is currently focusing so to solve for saccades you just build a small buffer area around the idealized minimum high res center and the saccades will safely stay inside that area within the ability of the system to react to the larger overall movements.

Picture demonstrating the large area that foveated rendering actually covers as high or mid res: https://www.reddit.com/r/oculus/comments/66nfap/made_a_pic_t...