So dtv2ser doesn't like my AVR-based boards, which use the cheap CH340 usb-serial chip. I fixed one driver bug but something else causes big transfers to fail pretty consistenly.
Since I'm stubborn and cheap, I didn't want to buy a 'proper' arduino, and I really didn't want to use a proper FTDI chip (after they deliberately bricked a huge number of people's devices they can die in a fire) so I ported the whole shebang over to an STM32F103 blue pill ($2 on ebay!) board I had lying around. It also knocked loose a bunch of other bugs in the client, so I fixed those up too.
It only took me ten years, but I can finally reflash my unit!