Your memory map seems alright.

I guess then it is more of a serial issue.

As I said before, BIOS shouldn't access the hardware when it is in the runtime phase after ExitBootServices. One approach is to pass the Null instance for the SerialLib as you suggested.  Make sure all the modules that are using SerialLib are using the NULL instance. For this you can check the Build report. I had the same issue and it seemed that the StatusCodeHandler Dxe Driver wasn't using the NULL instance for some reason. The Progress Status Codes were always a pain in the back and I never understood the point to it.

Anyways if that doesn't work for you either try to do the opposite and add debug logs to every function that is called during the ExitBootServices event and the Runtime event. I know it sounds unprofessional but unless you got some sort of hardware debugger working it's better than nothing.

Last but not least about the Linux debug logs you did build it correctly but you need to add some boot commands to redirect the logs to the serial port a quick search would get you going.

Hope that helps.

Best,

Alireza