Now I don't know how you feel about a wireless option, but if you are already using the 8bitdo wireless adapters, there is a wireless transmitter. It's made for the switch, and it uses batteries, but it works pretty well. I haven't tested it on the SNESC, though. The nunchuck also works for it as an NES controller without a start or select.
8Bitdo Gbros. Wireless Adapter for Nintendo Switch (Works with Wired GameCube & Classic Edition Controllers) - Nintendo Switch https://www.amazon.com/dp/B07JLF9PVG/ref=cm_sw_r_cp_apap_X45vZNJzxC8Nu
found this wireless adapter, may or may not be your taste: https://www.amazon.com/Wireless-Adapter-Nintendo-GameCube-Classic-Controllers/dp/B07JLF9PVG
i use 8bitdo gbros adapter. can plug in either gamecube controller or my snes mini controller. best part its also compatible with switch