Sizable puddle of brake fluid on ground underneath rear driver door

You are correct that this is most likely a brake line, and you should not drive the vehicle until you get it repaired.

There are a couple of options for this. If you find the leak which is going to be just above the place you find the fluid, then we can identify what type of line it is.

If this is already a flexible line, then that is just a simple replacement line.

However, if this is a hard line then we need to replace the entire line or at least splice in a new section if the rest of the line is okay.

Either way, you are most likely going to have to make a new line so here is a video on how to do this:

https://www.youtube.com/watch?v=sO43p4ZFUiM&t=7s

Below you will find the process from the manual on how to replace the line as well.

Please run through this info and let us know what you find.

Thanks