I’ve uploaded a fix to the variant pin definitions that gets Serial working w/ USART2. I had pins for USART3 as the default rx/tx the variants file but set serial 2 as the default… go figure. I tried using Serial3/USART3 on pins PB10 & PB11, but I haven’t found a way to get that working by editing the variants file or setting up Serial3 w/ HardwareSerial in main. I haven’t tried using USART1 or UART4 yet, so I don’t know if there’s any issues there either.
I also stopped being able to upload code using PlatformIO for some reason - maybe the linker script in there isn’t right since that’s copied from the STM32 ESC1, or the platformio.ini file needs upload arguments I don’t know about. I did find out that uploading built binaries with the STM32 ST-Link Utility is easy and that seems to work well, though.