How to structure UART code for portability, maintainability

I’ve implemented a UART communication project using a PIC microcontroller, where data is sent from a laptop to the MCU and received back on a serial terminal. Right now, the code is structured in separate files like main.c for application logic and uart.c / uart.h for driver-level implementation. This separation works fine for a small project.

Now I’m trying to think a bit ahead and improve the design so it becomes more reusable and portable for larger embedded systems. The code should be easy to maintain and extend. It should be reusable across different projects. It should be portable across different MCUs (for example, moving from 8-bit PIC to 32-bit PIC) with minimal changes

For example:

• If I want to change the baud rate (say from 9600 to 115200), I want it to be configurable in a simple way without touching multiple places in the code or rewriting logic. I’m also not fully sure whether this should be handled via a configuration structure or conditional compilation.
• If I migrate to a different PIC family, I want most of the application code to remain unchanged and only the hardware-specific UART should change.

Along with this, I also want to add a UART diagnostic or loopback test mode so that I can independently verify TX/RX communication. This would help debug UART issues even if the main application is not working properly.

I’m using a 8 bit PIC microcontroller and C language for this project. if you want, I can also share my code with result