nanoCH32V203 VS kmonad

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.

have you actually tried that? afaik they don't get you the perfect home row mods due to some limitations re. how they implement the tap vs hold logic

https://github.com/kmonad/kmonad/issues/228

I use kmonad[1] to have QMK-like functionality on any keyboard.

https://github.com/kmonad/kmonad

You can create F13-F24 purely in software with key mapping tools. On Windows, one way is with the PowerToys Keyboard Manager: remap some unimportant keys to F13, F14, etc. Another way is with KMonad (cross platform), and define the keymap with KeyF13, KeyF14, etc.

There you have the software link : https://github.com/kmonad/kmonad

Other option I would suggest for any linux user with keyboards without QMK is to try KMonad https://github.com/kmonad/kmonad