I'm doing a bit of re-wiring of my own machine and it's E-stop and am thinking along similar lines to you with regard to maintaining power to the controller (AXBB-E in my case) but cut power to the motor supplies. This will keep the UCCNC software running and make diagnostics and recovery simpler. I can't see a good reason for having separate contactors for each motor. As you say, if one stops you want them all to stop.

Something I am considering is a more accessible Process Stop button. Far more common for me than needing a quick E-stop is needing a quick process stop when I start the job and realise I haven't reset the tool-change interlock (the spindle won't start) or have forgotten to load the correct job file. It must be an age thing!