Max data transfer size/rate via USART?

Hi - sorry I'm a bit late to reply here. You really did have a good look in to it!

The output buffer is relatively small, yes - but it won't cause a crash or any kind of problem if that is overflowed - the function will just take a longer time to execute as it will wait for the buffer to become non-full.

For example:

Serial1.print("Hello"); // returns immediately
Serial1.print("Really long string .... ..."/* and so on */); // returns as soon as string length-128 characters have been transmitted (as then the rest is in the buffer)

Do you have up to date firmware? Does pressing Ctrl-C break out of it?

Please could you try and post up a really simple bit of code that crashes the Pico? Something I could just stick in the RHS of the IDE and try myself?

The only thing that immediately sticks out with what you're doing is:

var strRepresentation = JSON.stringify(obj);
Serial1.print(strRepresentation + '\u0004');

Adding a character on to the end of strRepresentation will cause a new variable to be created. If strRepresentation really is big then it could use up all your memory (but it still shouldn't crash - it should warn you of low memory).

To get around it you'd be better off doing 2 prints:

Serial1.print(strRepresentation);
Serial1.print('\u0004');