After trying a H7 (Orange Cube) I swapped the FC to a Pixhawk 4 Mini (F7).
While FrSky Passthrough works seamless on the F7, both with MAX3232 inverter and with SERIAL_OPTIONS 7 and straight wire, ESC protocols don’t. Regardless of MOT_PWM_TYPE, the output is plain PWM. BlHeli_32 ESCs will run, as they accept pwm controls, but APD ESCs blink communication error in DShot 300, 600 and 1200.