You have to know how kangaroo program works. A single V100 can solve a 64 bit key using kangaroo in mere seconds. One cannot use kangaroo to crack #64 now because the pub key is not known. But once someone broadcasts to transfer the BTC from #64's address, the pub key will be exposed and someone can use a single GPU to solve for the private key in seconds. That is what the OP is saying. As others have stated, using the RBF with decent fee, will help from others "stealing" #64s key.
Do you have a reference for saying that a single V100 can solve a 64 bit (of entropy) key in seconds?
If what you are saying is true, it would be advisable to
not use RBF. The RBF would need to be set to False and a decent fee that is sufficient to be included in the next block should be used.