What are they estimating the $600 for? The no communication issue is likely a fault in the ABS module. This issue presents itself similarly to the symptoms you mentioned. I’m not sure if GM used the same Electronic Brake Control Module on their sedans as they did on their trucks, but they were notorious for a common fault with broken solder joints. Here is a YouTube video of someone repairing it themselves. Again, this might not apply to your car but I’d look into it before spending the $600.
https://www.youtube.com/watch?v=R_5WZUebDx8