That gets done to what is the data
Is it a command?
Does it have a checksum or CRC? If so You need to wait till the last byte is in before you can move it.
Do you need the speed and RAM?
The developer has to do some work.
The "Rules" are not laws. They are guidelines, with plenty of exceptions. I
All the ways listed are good , bad or indifferent to some project.

In a recent project I need to respond to a serail command in 50ms. Noting else had a hard time, but did a lot of crunching.

My implementation Rx in the interupt to a ring Buffer. Process in main. State machine everything else so I could be in main to grab the bytes in time.

I should have process the serail in the Interupt. The crunching would have run striaght, no statics to hold values. But I followed the rules.
In the end it work fine, it was just more work and memory.
Know when to think out side the box, and when to stay in the lines. I am fairly sure the title Software Enginer means something.

My 2 cents