nanoCH32V203 VS ChibiOS-Contrib

If you are willing to try r/RISCV ones, then nanoCH32V203 (with two Full-Speed interfaces) and nanoCH32V305 (with one Full-Speed and one High-Speed interface) might be enough.

There are dual USB r/RISCV boards with GPIO pins exposed, like nanoCH32V203 and nanoCH32V305.

Another alternative is to use dual USB MCU board like nanoCH32V203 as USB CDC relay.

RISC-V boards cost $3-$6 depending if you want high speed USB or full speed one.

Their earlier boards like nanoCH32V203 and nanoCH32V305 were more user friendly, one could flash those via USB without WCH-Link.

Muse Lab has MCU boards like nanoCH32V203 and nanoCH32V305.

The same form factor as nanoCH32V203, but with more RAM and storage, also one of USB interfaces is high speed and SDIO is native this time. Comparable to CH32V307 boards, but lacks ethernet port.

Speaking of hardware there is very nice $3 RISC-V board that would fit well USB to USB converter use case. No keyboard firmware is ported to it yet.

First u/KarlK90 ported QMK, TMK and ChibiOS to GD32VF103 and used it in YAEMK.

Some of the work was upstreamed: https://github.com/ChibiOS/ChibiOS-Contrib/pull/248

Specifically for that MCU, there’s some initial support in ChibiOS-Contrib: https://github.com/ChibiOS/ChibiOS-Contrib/tree/chibios-20.3.x/os/hal/ports/LPC/LPC11Uxx

In regards to drivers for a processor or microcontroller, you may want to check out ChibiOS's HAL which provides non-blocking APIs. See here and here for an idea of what architectures are supported. I use this HAL whenever I can; it's clean and versatile in my opinion.