USBMidiKliK4x4 VS tinyusb

This DIY project would also work for you. https://github.com/TheKikGen/USBMidiKliK4x4

This Blue Pill based MIDI shield project might also be of interest.

Implementing the required USB host interface is another hurdle, but tinyusb seems to have sorted most of that out, except that UAC2 is implemented as a device while you need a host.

Have a look at TinyUSB. It supports the STM32WB and demo programs for the Nucleo-WB55RG. It even offers an example for dual CDC ports, so very close to what you're looking for.

Second: Distrust, misunderstanding, and out of date opinions of C++ in embedded spaces. I often see "compatibilty" thrown around as a goal for using C and not C++, but for most modern embedded systems, C++ compilers exist and are (usually) well tuned. Arm is a shining example here*. C++ is more optimizable than C as the intent can be clearer to the compiler. C++ constexpr and templates are a godsend for embedded systems, and I used both heavily in my recent arduino and pico testing. I combined both of them for a really cool experimental USB interface descriptor builder that runs at compile time and avoids the need to count byte sizes, but I gave up on it when the maintainer said C only, despite the fact that C can't do that. I was similarly disappointed when the pico "C++" was C only, as there was no backwards compatibility necessary.

I looked at this but the fork seems to have had a lot of problems from looking at the PR: https://github.com/hathach/tinyusb/pull/1219

I think a better solution might be to use a USB flash drive in the usbc port. Unfortunately micropython and circuitpython don't support this (yet), so you'd have to use c++ https://github.com/hathach/tinyusb

It's possible to make the s3 be a mass storage device via TinyUSB. https://github.com/hathach/tinyusb/tree/master/examples/device/cdc_msc_freertos