tinyusb VS lufa

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Second: Distrust, misunderstanding, and out of date opinions of C++ in embedded spaces.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

* QMK Toolbox 0.2.2 (https://qmk.fm/toolbox)* Supported bootloaders:* - ARM DFU (APM32, Kiibohd, STM32, STM32duino) via dfu-util (http://dfu-util.sourceforge.net/)* - Atmel/LUFA/QMK DFU via dfu-programmer (http://dfu-programmer.github.io/)* - Atmel SAM-BA (Massdrop) via Massdrop Loader (https://github.com/massdrop/mdloader)* - BootloadHID (Atmel, PS2AVRGB) via bootloadHID (https://www.obdev.at/products/vusb/bootloadhid.html)* - Caterina (Arduino, Pro Micro) via avrdude (http://nongnu.org/avrdude/)* - HalfKay (Teensy, Ergodox EZ) via Teensy Loader (https://pjrc.com/teensy/loader_cli.html)* - LUFA/QMK HID via hid_bootloader_cli (https://github.com/abcminiuser/lufa)* - LUFA Mass Storage* Supported ISP flashers:* - AVRISP (Arduino ISP)* - USBasp (AVR ISP)* - USBTiny (AVR Pocket)* Auto-flash enabled* Auto-flash disabled* Auto-flash enabledUSB device disconnected (CH341SER_A64): wch.cn USB-SERIAL CH340 (COM15) (1A86:7523:0264)USB device connected (CH341SER_A64): wch.cn USB-SERIAL CH340 (COM15) (1A86:7523:0264)

I briefly checked its code - wrapping it for reasonably convenient use with Arduino IDE would be not that simple. USB is quite abstract protocol and a particular device type requires a matching handler code. You can find example for keyboard HID here: https://github.com/abcminiuser/lufa/tree/master/Demos/Device/ClassDriver/Keyboard - as you can see, there is not so small amount of code for handling it: even though all really hard work is done inside the library, you still need to make a lot of proper calls to make it work.

For AVR devices I've always liked LUFA because it has tons of examples to get you started and works great even on low-end devices like the AT90USB162 (16MHz 8-bit cpu with 512 bytes of SRAM).