Hi @philpugh,
I remember a previous related thread about this:
I went through a lot of debugging with the other user but I originally didn't realize what @Patches ended up telling us, which was that there are two versions of the ARM architecture that Raspbian doesn't distinguish between properly. Thus, it's willing to install library packages that aren't applicable to the actual ARM hardware that you have on your Pi. That leads to these illegal instruction errors.
I'm not sure that I understood in the other thread what people are recommended to do about this situation.