STM32H723: Can't connect to bootloader via UART / at all

I am trying to use the built-in bootloader to flash a new firmware onto my MCU (STM32H723). Unfortunately, I cannot get any reaction from the MCU when trying to connect to it, and I am struggling to diagnose which step is going wrong.

Short version of my steps:

• I pull BOOT0 high & NRST low (before this point, the MCU is running the normal firmware code)
• I release NRST, the MCU starts up again (attaching the ST-Link debugger, I confirmed that the code being execute is around 0x1FF00000, i.e. the MCU is somewhere in the bootloader code)
• I try to connect to the MCU using the STM32CubeProgrammer, set to UART (MCU pins PA9/PA10), baud 9600 (tried everything up to 115200), even parity, 8 data bits, 1 stop bit, flow control off. Whatever settings I try, I always get:

Port configuration: parity = even, baudrate = 9600, data-bit = 8, stop-bit = 1.0, flow-control = off
Timeout error occured while waiting for acknowledgement.
Error: Activating device: KO. Please, verify the boot mode configuration and check the serial port configuration. Reset your device then try again... ​

I am struggling to figure out what's going wrong: Reading the bootloader documentation (AN2606), I get the impression that all the peripherals should by default be disabled, and only once it detects the "activation sequence" on one of them will that one be enabled. So I tried to check with the debugger to see which peripherals are enabled (to maybe figure out which other peripheral is erroneously selected instead of UART): For some reason, most of the relevant peripherals (UART1,2,3 I2C1,2,3, etc) are all simultaneously enabled.

What could I be doing wrong? How can I diagnose this?

More details:

• The MCU is connected to the PC as follows: FT232HQ chip -> Optical isolator (for UART & BOOT0 & NRST) -> MCU (pins BOOT0, NRST, PA9, PA10)
• I pretty sure I am controlling the BOOT0 & NRST pins correctly, given that the MCU is executing code in the 0x1FF00000 range when BOOT0 is high, and 0x08000000 when BOOT0 is not high
• Using a logic analyzer, I can see the 0x7F byte being sent from the PC on the RX pin of the MCU, so the UART communication towards the MCU seems fine. I cannot see anything on the TX pin, so the MCU doesn't seem to ever react.