Not completely sure I got this thing about the extra iC required, or maybe my suggestion required some more elaboration.
Your MCU probably has a spare UART connection available? That's just two GPIOs. I understand that if you're using a small MCU with 100% of GPIOs already used, that may be an issue, and in this case you have to become a bit more creative.
Otherwise, just route spare UART pins to a small connector of your liking (can also just be pads and a pogo pin adapter if PCB area is limited), add some minimal protection (like TVS diodes and/or series resistors) if you really want to be super safe.
And then use any off-the-shelf USB-to-UART adapter, that are available in more shapes, forms and variants than you can think of. Absolutely doesn't need any RS232 level shifting, I would actually not recommend it at all if this is a production interface not for the end-user. Just use TTL levels. To make things even better and more flexible, buy an isolated USB-to-UART adapter, those provide isolaiton from USB (which can be a plus) and usually allow providing the target ref. voltage, so you get level shifting as well (useful if your target level is not typical 3.3V.) This will be part of your production tools.