nanoCH32V203 VS yaemk-split-kb

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.

yaemk

The YAEMK is one I've been considering myself lately, but you've got a lot of options for split boards. You can look at and directly compare a lot of them here. If you want choc switches, without going to the sub 40% size range then the main options it looks like you have are:

There is the YAEMK

It would probably be cleaner to just put silicone stickers on a regular case like here: https://karlk90.github.io/yaemk-split-kb/

I was actually most intrigued by the Yaemk, but I think it's a too much DIY for a beginner such as I (no kits available AFAIK, has a disclaimer that it can be challenging for someone with no soldering experience).

My YAEMK supports both ARM and RISC-V MCUs. Schematics, PCB and an extensive build guide is available here -> https://karlk90.github.io/yaemk-split-kb/

Here is his YAEMK, a split, ergo keyboard that can use the same MCU, or an ARM MCU: https://karlk90.github.io/yaemk-split-kb/