Amazing! This is the easiest C422 system I've seen yet. You may then change to OpenCore release, and add the OpenCanopy GUI if you like.
If you do post the boot log and SysReport folder I may still see if one could further optimise the EFI folder.
For USB you have to do the mapping yourself. Running "in production" with the port limit quirk is dangerous because it can cause buffer overflows. Follow here with CorpNewt' USBMap (read the introductory pages first)
If possible use a USB3 hub—this plugs to both USB2 and USB3 personalities, and does not need to be ejected. Else use both a USB2 and a USB3 device to find out everything. Test USB-C ports in both orientations (this is important!). Make a sketch or a table of everything with the USB personalities (HS## for USB2, SS# for USB3) and the hexadecimal port number 0x## for each (important!). Don't forget the SD reader!
If all ports come from the chipset, which is likely, you'll need to make choices to remain within the 15 ports limit. Tough choices if Dell went for the cheap solution and made type 10 USB-C ports rather than type 9.
ALC3234 is ALC255, so layout 1 does not even exist. (I could have checked that beforehand.) Edit the config.plist, change boot argument "alcid=1" to "alcid=3", reboot and test all sound inputs and outputs. Repeat with each possible layout: 3, 11, 12, 13, 15, 17, 18, 20, 21, 27, 28, 29, 30, 31, 66, 71, 82, 86, 96, 99, 100. It's tedious but it doubles down as a good test that reboot is stable.
When you have tested all, make the best layout permanent, as boot-arg or in the DeviceProperties section.
No, there can only be one layout; it's a global setting. You really have to try all layouts one by one; for each layout, try all outputs one by one in SystemPreferences>Sound (and keep moving your speaker from port to port if you only have one), note what what works, what does not; and pick up the best overall layout at the end. (Murphy's Law: No matter in which order you try, the best setting will come out last.)
There's no way to help you remotely. Only someone who has been through this procedure with the very same motherboard could tell you how Dell has mapped the lines of the audio chip here.
At least you're not trying to map a 7.1 panel with just one speaker…
And this only has to be done once and for all.
About USB mapping: Recent versions of Big Sur reportedly broke the process. If you've installed an early version, it should be fine. Otherwise, it may be easier to install Catalina and map there. Again, it's a "once and for all" affair and the USB map is valid in any OS X version.
You're welcome! I didn't do much, and cannot help with sound, but if you do post the SysReport folder and a bootlog I can have a look and see what could be improved. (Admittedly, I'm also curious to see what a native NVRAM implementation on C422 looks like.)
Do not forget to switch to OpenCore release, disable SysReport and lower Debug>Target to 3 or less. It will speed up boot time, if only a little.
Edit. If your USBMap includes some USB-C port, they should be type 9 or 10 rather than 3.