There are no easy answers, no low hanging fruit.
From an engineering standpoint, it is unacceptable considering that there are too many unknown "vectors." So much as the hole to the mantle is concerned, it doesn't make sense cost wise as the hole would cost more (far) than a traditional reprocessing means (whatever that may be).
This is one reason a huge amount has been spent on fusion energy and that incredibly exotic technology. Smaller reactors like Bill Gates travelling wave reactor are promising, but yet unproven technology.
Probably the way forward over huge monolithic reactors from the past - which can not be built unless government doles out money to them. Because the free market does not accept them on their own merits.
As you aptly said, no low hanging fruit that can be seen.