The Radeon Custom Resolution setting's Hz adjustment didn't stick at all, and while CRU allowed the additional Hz options to show up in Window 10's Monitor Setting section, the Dell U2412M monitor itself seems to be resisting it. I have installed the driver for the monitor, but that didn't help.
Error message from the monitor when I applied 65 or 61 Hz and the monitor reboots: "The current input timing is not supported by monitor display. Please change your input timing to 1920x1200 60Hz."
Not sure if there's a way around the monitor's firmware issue or if I'll just have to accept the 60 Hz. I got the monitor for free so I shouldn't be complaining about it.