"A toxic legacy measured not in decades but centuries"?
"But planting things that take at least 20 years to mature does not strike me as biologically unsustainable as, say, oh I dunno, building more nuclear power plants..."
The thing about nuclear waste is that it is produced in small amounts and it is increasingly well handled through new technologies. For example, Gen III+ and Gen IV reactors will transmute a large proportion of transuranic waste, leaving it with a much reduced half-life and providing energy in the conversion process as a bonus.
By comparison, how long will it be before the doubling of mercury in the world's oceans returns to its pre-industrial level? Similarly, it would be interesting to know how much radioactivity will be released in the next two decades from the production and use of phosphorus based fertiliser.