As a geologist myself, it doesnt sound like you are well informed. There are plenty of regions on the globe that haven't exhibited any real geological activity for billions of years. The places that deep geological repositories are proposed are very deep as in far below the water table and in impermeable rock. Erosion is neglible in these areas, and there are very few geological processes that could conceivably change that. The waste itself is to be stored in multiple layers of protection, right down to the material the waste is composed of, which has a low water solubility.

Is it possible that a mid contental rift will open up near one of these and result in processes that ruin the storage site? Sure, but thats so unlikely that we might as well start talking about a big meteor crashing into the site and spraying nuclear fallout across the planet (which would kind of happen anyways with a meteor that large). Point is, the risk of that happening even on geological timescale is pretty low. There are larger risks associated with natural uranium deposits or even regions with large amounts of granite.

The biggest risks of DGRs is that some time in the future, humans forget about where they are or cant understand the warnings placed on them and accidentally dig them up before they decay enough.