Why dont you suggest what "safe" looks like, and we can discuss your understanding of safety. Its clear to me that the issue is your standards and not actual waste disposal.

My understanding is that this material remains toxic to life for thousands, to tens of thousands of years.

Safe means that it's stored such that there's no harm to the environment for that lifetime.

In all "bury it" scenarios, the place where the waste is buried will be subject to change resulting in water, air, able to interact with that waste when normal tectonic and erosion processes do their thing.