My understanding of this is as follows:
As you have indicated in your question, there is not a clearcut difference between "pared" and "muro", except that "muro" can apply to a freestanding wall (though that is not relevant here), and that a partition wall within a building is more likely to be called a "pared" than a "muro". Applied to the wall of a building, "pared" is a more common word in general. In a building a "muro" almost always denotes a load-bearing wall, and is most likely to apply to external walls. Yet these can be called "paredes" too, and "pared maestra" is common usage. Generally speaking, for the same reasons, you would expect muros to be thicker than paredes.

So one option is just to say "walls" and leave it at that. However, if "las paredes o los muros" is not a mere pleonasm, but the words are being differentiated from each other in some way, I think that difference must be between internal and external walls, and that these are definitely the terms that should be used in English. Always on the understanding that it is not true that "pared" can only mean an internal wall.