nanoCH32V203 VS riscv-isa-sim

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.

Someone correct me if I'm wrong, but Spike is considered the gold standard for RISV-V simulation, in terms of support for extensions and overall correctness. As I understand it, QEMU is faster and easier to use for day-to-day for general software development.

Spike is an RISC-V instruction set simulator: https://github.com/riscv-software-src/riscv-isa-sim

I really like the source code for the Spike RISC-V ISA Simulator. It's not very heavily commented, though, so you really need to read the code.

If you want to run bare metal with no RTOS, it should be possible, but you will need to replace the main startup program (https://github.com/riscv-software-src/riscv-isa-sim/blob/master/spike_main/spike.cc) with some program to set up the hardware and instantiate the simulator, load the OS image etc and then have a decent runtime environment to that supports malloc() etc and redirect IO to serial or flash memory etc. There is a bit of work you would need to do.

I guess you will also use Spike and the Sail model for RISC-V.